zig/src/InternPool.zig

//! All interned objects have both a value and a type.
//! This data structure is self-contained.
const InternPool = @This();

const builtin = @import("builtin");
const build_options = @import("build_options");

const std = @import("std");
const Io = std.Io;
const Allocator = std.mem.Allocator;
const assert = std.debug.assert;
const BigIntConst = std.math.big.int.Const;
const BigIntMutable = std.math.big.int.Mutable;
const Cache = std.Build.Cache;
const Limb = std.math.big.Limb;
const Hash = std.hash.Wyhash;
const Zir = std.zig.Zir;

const Zcu = @import("Zcu.zig");
const TypeClass = @import("Type.zig").Class;

/// One item per thread, indexed by `tid`, which is dense and unique per thread.
locals: []Local,
/// Length must be a power of two and represents the number of simultaneous
/// writers that can mutate any single sharded data structure.
shards: []Shard,
/// Key is the error name, index is the error tag value. Index 0 has a length-0 string.
global_error_set: GlobalErrorSet,
/// Cached number of active bits in a `tid`.
tid_width: if (single_threaded) u0 else std.math.Log2Int(u32),
/// Cached shift amount to put a `tid` in the top bits of a 30-bit value.
tid_shift_30: if (single_threaded) u0 else std.math.Log2Int(u32),
/// Cached shift amount to put a `tid` in the top bits of a 31-bit value.
tid_shift_31: if (single_threaded) u0 else std.math.Log2Int(u32),
/// Cached shift amount to put a `tid` in the top bits of a 32-bit value.
tid_shift_32: if (single_threaded) u0 else std.math.Log2Int(u32),

/// Dependencies on the source code hash associated with a ZIR instruction.
/// * For a `declaration`, this is the entire declaration body.
/// * For a `struct_decl`, `union_decl`, etc, this is the source of the fields (but not declarations).
/// * For a `func`, this is the source of the full function signature.
/// These are also invalidated if tracking fails for this instruction.
/// Value is index into `dep_entries` of the first dependency on this hash.
src_hash_deps: std.AutoArrayHashMapUnmanaged(TrackedInst.Index, DepEntry.Index),
/// Dependencies on the value of a Nav.
/// Value is index into `dep_entries` of the first dependency on this Nav value.
nav_val_deps: std.AutoArrayHashMapUnmanaged(Nav.Index, DepEntry.Index),
/// Dependencies on the type of a Nav.
/// Value is index into `dep_entries` of the first dependency on this Nav value.
nav_ty_deps: std.AutoArrayHashMapUnmanaged(Nav.Index, DepEntry.Index),
/// Dependencies on a function's inferred error set. Key is the function body, not the IES.
/// Value is index into `dep_entries` of the first dependency on this function's IES.
func_ies_deps: std.AutoArrayHashMapUnmanaged(Index, DepEntry.Index),
/// Dependencies on the resolved layout of a `struct`, `union`, or `enum` type.
/// Value is index into `dep_entries` of the first dependency on this type's layout.
type_layout_deps: std.AutoArrayHashMapUnmanaged(Index, DepEntry.Index),
/// Dependencies on a ZON file. Triggered by `@import` of ZON.
/// Value is index into `dep_entries` of the first dependency on this ZON file.
zon_file_deps: std.AutoArrayHashMapUnmanaged(FileIndex, DepEntry.Index),
/// Dependencies on an embedded file.
/// Introduced by `@embedFile`; invalidated when the file changes.
/// Value is index into `dep_entries` of the first dependency on this `Zcu.EmbedFile`.
embed_file_deps: std.AutoArrayHashMapUnmanaged(Zcu.EmbedFile.Index, DepEntry.Index),
/// Dependencies on the full set of names in a ZIR namespace.
/// Key refers to a `struct_decl`, `union_decl`, etc.
/// Value is index into `dep_entries` of the first dependency on this namespace.
namespace_deps: std.AutoArrayHashMapUnmanaged(TrackedInst.Index, DepEntry.Index),
/// Dependencies on the (non-)existence of some name in a namespace.
/// Value is index into `dep_entries` of the first dependency on this name.
namespace_name_deps: std.AutoArrayHashMapUnmanaged(NamespaceNameKey, DepEntry.Index),
// Dependencies on the value of fields memoized on `Zcu` (`panic_messages` etc).
// If set, these are indices into `dep_entries` of the first dependency on this state.
memoized_state_main_deps: DepEntry.Index.Optional,
memoized_state_panic_deps: DepEntry.Index.Optional,
memoized_state_va_list_deps: DepEntry.Index.Optional,
memoized_state_assembly_deps: DepEntry.Index.Optional,

/// Given a `Depender`, points to an entry in `dep_entries` whose `depender`
/// matches. The `next_dependee` field can be used to iterate all such entries
/// and remove them from the corresponding lists.
first_dependency: std.AutoArrayHashMapUnmanaged(AnalUnit, DepEntry.Index),

/// Stores dependency information. The hashmaps declared above are used to look
/// up entries in this list as required. This is not stored in `extra` so that
/// we can use `free_dep_entries` to track free indices, since dependencies are
/// removed frequently.
dep_entries: std.ArrayList(DepEntry),
/// Stores unused indices in `dep_entries` which can be reused without a full
/// garbage collection pass.
free_dep_entries: std.ArrayList(DepEntry.Index),

/// Whether a single-threaded intern pool impl is in use.
pub const single_threaded = switch (build_options.io_mode) {
    .threaded => builtin.single_threaded,
    .evented => false, // even without threads, evented can be access from multiple tasks at a time
};

pub const empty: InternPool = .{
    .locals = &.{},
    .shards = &.{},
    .global_error_set = .empty,
    .tid_width = 0,
    .tid_shift_30 = if (single_threaded) 0 else 31,
    .tid_shift_31 = if (single_threaded) 0 else 31,
    .tid_shift_32 = if (single_threaded) 0 else 31,
    .src_hash_deps = .empty,
    .nav_val_deps = .empty,
    .nav_ty_deps = .empty,
    .func_ies_deps = .empty,
    .type_layout_deps = .empty,
    .zon_file_deps = .empty,
    .embed_file_deps = .empty,
    .namespace_deps = .empty,
    .namespace_name_deps = .empty,
    .memoized_state_main_deps = .none,
    .memoized_state_panic_deps = .none,
    .memoized_state_va_list_deps = .none,
    .memoized_state_assembly_deps = .none,
    .first_dependency = .empty,
    .dep_entries = .empty,
    .free_dep_entries = .empty,
};

/// A `TrackedInst.Index` provides a single, unchanging reference to a ZIR instruction across a whole
/// compilation. From this index, you can acquire a `TrackedInst`, which containss a reference to both
/// the file which the instruction lives in, and the instruction index itself, which is updated on
/// incremental updates by `Zcu.updateZirRefs`.
pub const TrackedInst = extern struct {
    file: FileIndex,
    inst: Zir.Inst.Index,

    /// It is possible on an incremental update that we "lose" a ZIR instruction: some tracked `%x` in
    /// the old ZIR failed to map to any `%y` in the new ZIR. For this reason, we actually store values
    /// of type `MaybeLost`, which uses `ZirIndex.lost` to represent this case. `Index.resolve` etc
    /// return `null` when the `TrackedInst` being resolved has been lost.
    pub const MaybeLost = extern struct {
        file: FileIndex,
        inst: ZirIndex,
        pub const ZirIndex = enum(u32) {
            /// Tracking failed for this ZIR instruction. Uses of it should fail.
            lost = std.math.maxInt(u32),
            _,
            pub fn unwrap(inst: ZirIndex) ?Zir.Inst.Index {
                return switch (inst) {
                    .lost => null,
                    _ => @enumFromInt(@intFromEnum(inst)),
                };
            }
            pub fn wrap(inst: Zir.Inst.Index) ZirIndex {
                return @enumFromInt(@intFromEnum(inst));
            }
        };
        comptime {
            // The fields should be tightly packed. See also serialiation logic in `Compilation.saveState`.
            assert(@sizeOf(@This()) == @sizeOf(FileIndex) + @sizeOf(ZirIndex));
        }
    };

    pub const Index = enum(u32) {
        _,
        pub fn resolveFull(tracked_inst_index: TrackedInst.Index, ip: *const InternPool) ?TrackedInst {
            const tracked_inst_unwrapped = tracked_inst_index.unwrap(ip);
            const tracked_insts = ip.getLocalShared(tracked_inst_unwrapped.tid).tracked_insts.acquire();
            const maybe_lost = tracked_insts.view().items(.@"0")[tracked_inst_unwrapped.index];
            return .{
                .file = maybe_lost.file,
                .inst = maybe_lost.inst.unwrap() orelse return null,
            };
        }
        pub fn resolveFile(tracked_inst_index: TrackedInst.Index, ip: *const InternPool) FileIndex {
            const tracked_inst_unwrapped = tracked_inst_index.unwrap(ip);
            const tracked_insts = ip.getLocalShared(tracked_inst_unwrapped.tid).tracked_insts.acquire();
            const maybe_lost = tracked_insts.view().items(.@"0")[tracked_inst_unwrapped.index];
            return maybe_lost.file;
        }
        pub fn resolve(i: TrackedInst.Index, ip: *const InternPool) ?Zir.Inst.Index {
            return (i.resolveFull(ip) orelse return null).inst;
        }

        pub fn toOptional(i: TrackedInst.Index) Optional {
            return @enumFromInt(@intFromEnum(i));
        }
        pub const Optional = enum(u32) {
            none = std.math.maxInt(u32),
            _,
            pub fn unwrap(opt: Optional) ?TrackedInst.Index {
                return switch (opt) {
                    .none => null,
                    _ => @enumFromInt(@intFromEnum(opt)),
                };
            }

            const debug_state = InternPool.debug_state;
        };

        pub const Unwrapped = struct {
            tid: Zcu.PerThread.Id,
            index: u32,

            pub fn wrap(unwrapped: Unwrapped, ip: *const InternPool) TrackedInst.Index {
                assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
                assert(unwrapped.index <= ip.getIndexMask(u32));
                return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                    unwrapped.index);
            }
        };
        pub fn unwrap(tracked_inst_index: TrackedInst.Index, ip: *const InternPool) Unwrapped {
            return .{
                .tid = @enumFromInt(@intFromEnum(tracked_inst_index) >> ip.tid_shift_32 & ip.getTidMask()),
                .index = @intFromEnum(tracked_inst_index) & ip.getIndexMask(u32),
            };
        }

        const debug_state = InternPool.debug_state;
    };
};

pub fn trackZir(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    key: TrackedInst,
) Allocator.Error!TrackedInst.Index {
    const maybe_lost_key: TrackedInst.MaybeLost = .{
        .file = key.file,
        .inst = TrackedInst.MaybeLost.ZirIndex.wrap(key.inst),
    };
    const full_hash = Hash.hash(0, std.mem.asBytes(&maybe_lost_key));
    const hash: u32 = @truncate(full_hash >> 32);
    const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
    var map = shard.shared.tracked_inst_map.acquire();
    const Map = @TypeOf(map);
    var map_mask = map.header().mask();
    var map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.acquire().unwrap() orelse break;
        if (entry.hash != hash) continue;
        if (std.meta.eql(index.resolveFull(ip) orelse continue, key)) return index;
    }
    shard.mutate.tracked_inst_map.mutex.lock(io, tid);
    defer shard.mutate.tracked_inst_map.mutex.unlock(io);
    if (map.entries != shard.shared.tracked_inst_map.entries) {
        map = shard.shared.tracked_inst_map;
        map_mask = map.header().mask();
        map_index = hash;
    }
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.acquire().unwrap() orelse break;
        if (entry.hash != hash) continue;
        if (std.meta.eql(index.resolveFull(ip) orelse continue, key)) return index;
    }
    defer shard.mutate.tracked_inst_map.len += 1;
    const local = ip.getLocal(tid);
    const list = local.getMutableTrackedInsts(gpa, io);
    try list.ensureUnusedCapacity(1);
    const map_header = map.header().*;
    if (shard.mutate.tracked_inst_map.len < map_header.capacity * 3 / 5) {
        const entry = &map.entries[map_index];
        entry.hash = hash;
        const index = (TrackedInst.Index.Unwrapped{
            .tid = tid,
            .index = list.mutate.len,
        }).wrap(ip);
        list.appendAssumeCapacity(.{maybe_lost_key});
        entry.release(index.toOptional());
        return index;
    }
    const arena_state = &local.mutate.arena;
    var arena = arena_state.promote(gpa);
    defer arena_state.* = arena.state;
    const new_map_capacity = map_header.capacity * 2;
    const new_map_buf = try arena.allocator().alignedAlloc(
        u8,
        .fromByteUnits(Map.alignment),
        Map.entries_offset + new_map_capacity * @sizeOf(Map.Entry),
    );
    const new_map: Map = .{ .entries = @ptrCast(new_map_buf[Map.entries_offset..].ptr) };
    new_map.header().* = .{ .capacity = new_map_capacity };
    @memset(new_map.entries[0..new_map_capacity], .{ .value = .none, .hash = undefined });
    const new_map_mask = new_map.header().mask();
    map_index = 0;
    while (map_index < map_header.capacity) : (map_index += 1) {
        const entry = &map.entries[map_index];
        const index = entry.value.unwrap() orelse continue;
        const item_hash = entry.hash;
        var new_map_index = item_hash;
        while (true) : (new_map_index += 1) {
            new_map_index &= new_map_mask;
            const new_entry = &new_map.entries[new_map_index];
            if (new_entry.value != .none) continue;
            new_entry.* = .{
                .value = index.toOptional(),
                .hash = item_hash,
            };
            break;
        }
    }
    map = new_map;
    map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= new_map_mask;
        if (map.entries[map_index].value == .none) break;
    }
    const index = (TrackedInst.Index.Unwrapped{
        .tid = tid,
        .index = list.mutate.len,
    }).wrap(ip);
    list.appendAssumeCapacity(.{maybe_lost_key});
    map.entries[map_index] = .{ .value = index.toOptional(), .hash = hash };
    shard.shared.tracked_inst_map.release(new_map);
    return index;
}

/// At the start of an incremental update, we update every entry in `tracked_insts` to include
/// the new ZIR index. Once this is done, we must update the hashmap metadata so that lookups
/// return correct entries where they already exist.
pub fn rehashTrackedInsts(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
) Allocator.Error!void {
    assert(tid == .main); // we shouldn't have any other threads active right now

    // TODO: this function doesn't handle OOM well. What should it do?

    // We don't lock anything, as this function assumes that no other thread is
    // accessing `tracked_insts`. This is necessary because we're going to be
    // iterating the `TrackedInst`s in each `Local`, so we have to know that
    // none will be added as we work.

    // Figure out how big each shard need to be and store it in its mutate `len`.
    for (ip.shards) |*shard| shard.mutate.tracked_inst_map.len = 0;
    for (ip.locals) |*local| {
        // `getMutableTrackedInsts` is okay only because no other thread is currently active.
        // We need the `mutate` for the len.
        for (local.getMutableTrackedInsts(gpa, io).viewAllowEmpty().items(.@"0")) |tracked_inst| {
            if (tracked_inst.inst == .lost) continue; // we can ignore this one!
            const full_hash = Hash.hash(0, std.mem.asBytes(&tracked_inst));
            const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
            shard.mutate.tracked_inst_map.len += 1;
        }
    }

    const Map = Shard.Map(TrackedInst.Index.Optional);

    const arena_state = &ip.getLocal(tid).mutate.arena;

    // We know how big each shard must be, so ensure we have the capacity we need.
    for (ip.shards) |*shard| {
        const want_capacity = if (shard.mutate.tracked_inst_map.len == 0) 0 else cap: {
            // We need to return a capacity of at least 2 to make sure we don't have the `Map(...).empty` value.
            // For this reason, note the `+ 1` in the below expression. This matches the behavior of `trackZir`.
            break :cap std.math.ceilPowerOfTwo(u32, shard.mutate.tracked_inst_map.len * 5 / 3 + 1) catch unreachable;
        };
        const have_capacity = shard.shared.tracked_inst_map.header().capacity; // no acquire because we hold the mutex
        if (have_capacity >= want_capacity) {
            if (have_capacity == 1) {
                // The map is `.empty` -- we can't memset the entries, or we'll segfault, because
                // the buffer is secretly constant.
            } else {
                @memset(shard.shared.tracked_inst_map.entries[0..have_capacity], .{ .value = .none, .hash = undefined });
            }
            continue;
        }
        var arena = arena_state.promote(gpa);
        defer arena_state.* = arena.state;
        const new_map_buf = try arena.allocator().alignedAlloc(
            u8,
            .fromByteUnits(Map.alignment),
            Map.entries_offset + want_capacity * @sizeOf(Map.Entry),
        );
        const new_map: Map = .{ .entries = @ptrCast(new_map_buf[Map.entries_offset..].ptr) };
        new_map.header().* = .{ .capacity = want_capacity };
        @memset(new_map.entries[0..want_capacity], .{ .value = .none, .hash = undefined });
        shard.shared.tracked_inst_map.release(new_map);
    }

    // Now, actually insert the items.
    for (ip.locals, 0..) |*local, local_tid| {
        // `getMutableTrackedInsts` is okay only because no other thread is currently active.
        // We need the `mutate` for the len.
        for (local.getMutableTrackedInsts(gpa, io).viewAllowEmpty().items(.@"0"), 0..) |tracked_inst, local_inst_index| {
            if (tracked_inst.inst == .lost) continue; // we can ignore this one!
            const full_hash = Hash.hash(0, std.mem.asBytes(&tracked_inst));
            const hash: u32 = @truncate(full_hash >> 32);
            const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
            const map = shard.shared.tracked_inst_map; // no acquire because we hold the mutex
            const map_mask = map.header().mask();
            var map_index = hash;
            const entry = while (true) : (map_index += 1) {
                map_index &= map_mask;
                const entry = &map.entries[map_index];
                if (entry.acquire() == .none) break entry;
            };
            const index = TrackedInst.Index.Unwrapped.wrap(.{
                .tid = @enumFromInt(local_tid),
                .index = @intCast(local_inst_index),
            }, ip);
            entry.hash = hash;
            entry.release(index.toOptional());
        }
    }
}

/// Analysis Unit. Represents a single entity which undergoes semantic analysis.
/// This is the "source" of an incremental dependency edge.
pub const AnalUnit = packed struct(u64) {
    kind: Kind,
    id: u32,

    pub const Kind = enum(u32) {
        @"comptime",
        nav_val,
        nav_ty,
        type_layout,
        func,
        memoized_state,
    };

    pub const Unwrapped = union(Kind) {
        /// This `AnalUnit` analyzes the body of the given `comptime` declaration.
        @"comptime": ComptimeUnit.Id,
        /// This `AnalUnit` resolves the value of the given `Nav`.
        nav_val: Nav.Index,
        /// This `AnalUnit` resolves the type of the given `Nav`.
        nav_ty: Nav.Index,
        /// This `AnalUnit` resolves the layout of the given `struct`, `union`, or `enum` type.
        type_layout: InternPool.Index,
        /// This `AnalUnit` analyzes the body of the given runtime function.
        func: InternPool.Index,
        /// This `AnalUnit` resolves all state which is memoized in fields on `Zcu`.
        memoized_state: MemoizedStateStage,
    };

    pub fn unwrap(au: AnalUnit) Unwrapped {
        return switch (au.kind) {
            inline else => |tag| @unionInit(
                Unwrapped,
                @tagName(tag),
                @enumFromInt(au.id),
            ),
        };
    }
    pub fn wrap(raw: Unwrapped) AnalUnit {
        return switch (raw) {
            inline else => |id, tag| .{
                .kind = tag,
                .id = @intFromEnum(id),
            },
        };
    }

    pub fn toOptional(as: AnalUnit) Optional {
        return @enumFromInt(@as(u64, @bitCast(as)));
    }
    pub const Optional = enum(u64) {
        none = std.math.maxInt(u64),
        _,
        pub fn unwrap(opt: Optional) ?AnalUnit {
            return switch (opt) {
                .none => null,
                _ => @bitCast(@intFromEnum(opt)),
            };
        }
    };
};

pub const MemoizedStateStage = enum(u32) {
    /// Everything other than panics and `VaList`.
    main,
    /// Everything within `std.builtin.Panic`.
    /// Since the panic handler is user-provided, this must be able to reference the other memoized state.
    panic,
    /// Specifically `std.builtin.VaList`. See `Zcu.BuiltinDecl.stage`.
    va_list,
    /// Everything within `std.builtin.assembly`. See `Zcu.BuiltinDecl.stage`.
    assembly,
};

pub const ComptimeUnit = extern struct {
    zir_index: TrackedInst.Index,
    namespace: NamespaceIndex,

    comptime {
        assert(std.meta.hasUniqueRepresentation(ComptimeUnit));
    }

    pub const Id = enum(u32) {
        _,
        const Unwrapped = struct {
            tid: Zcu.PerThread.Id,
            index: u32,
            fn wrap(unwrapped: Unwrapped, ip: *const InternPool) ComptimeUnit.Id {
                assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
                assert(unwrapped.index <= ip.getIndexMask(u32));
                return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                    unwrapped.index);
            }
        };
        fn unwrap(id: Id, ip: *const InternPool) Unwrapped {
            return .{
                .tid = @enumFromInt(@intFromEnum(id) >> ip.tid_shift_32 & ip.getTidMask()),
                .index = @intFromEnum(id) & ip.getIndexMask(u31),
            };
        }

        const debug_state = InternPool.debug_state;
    };
};

/// Named Addressable Value. Represents a global value with a name and address. This name may be
/// generated, and the type (and hence address) may be comptime-only. A `Nav` whose type has runtime
/// bits is sent to the linker to be emitted to the binary.
///
/// * Every ZIR `declaration` which is not a `comptime` declaration has a `Nav` (post-instantiation)
///   which stores the declaration's resolved value.
/// * Generic instances have a `Nav` corresponding to the instantiated function.
/// * `@extern` calls create a `Nav` whose value is a `.@"extern"`.
///
/// This data structure is optimized for the `analysis_info != null` case, because this is much more
/// common in practice; the other case is used only for externs and for generic instances. At the time
/// of writing, in the compiler itself, around 74% of all `Nav`s have `analysis_info != null`.
/// (Specifically, 104225 / 140923)
///
/// `Nav.Repr` is the in-memory representation.
pub const Nav = struct {
    /// The unqualified name of this `Nav`. Namespace lookups use this name, and error messages may use it.
    /// Additionally, extern `Nav`s (i.e. those whose value is an `extern`) use this name.
    name: NullTerminatedString,
    /// The fully-qualified name of this `Nav`.
    fqn: NullTerminatedString,
    /// This field is populated iff this `Nav` is resolved by semantic analysis.
    /// If this is `null`, then `status == .fully_resolved` always.
    analysis: ?struct {
        namespace: NamespaceIndex,
        zir_index: TrackedInst.Index,
        /// Initially `false`. Set to `true` by `setWantNavAnalysis`.
        wanted: bool,
    },
    status: union(enum) {
        /// This `Nav` is pending semantic analysis.
        unresolved,
        /// The type of this `Nav` is resolved; the value is queued for resolution.
        type_resolved: struct {
            type: InternPool.Index,
            is_const: bool,
            alignment: Alignment,
            @"linksection": OptionalNullTerminatedString,
            @"addrspace": std.builtin.AddressSpace,
            is_threadlocal: bool,
            /// This field is whether this `Nav` is a literal `extern` definition.
            /// It does *not* tell you whether this might alias an extern fn (see #21027).
            is_extern_decl: bool,
        },
        /// The value of this `Nav` is resolved.
        fully_resolved: struct {
            val: InternPool.Index,
            is_const: bool,
            alignment: Alignment,
            @"linksection": OptionalNullTerminatedString,
            @"addrspace": std.builtin.AddressSpace,
        },
    },

    /// Asserts that `status != .unresolved`.
    pub fn typeOf(nav: Nav, ip: *const InternPool) InternPool.Index {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| r.type,
            .fully_resolved => |r| ip.typeOf(r.val),
        };
    }

    /// This function is intended to be used by code generation, since semantic
    /// analysis will ensure that any `Nav` which is potentially `extern` is
    /// fully resolved.
    /// Asserts that `status == .fully_resolved`.
    pub fn getResolvedExtern(nav: Nav, ip: *const InternPool) ?Key.Extern {
        assert(nav.status == .fully_resolved);
        return nav.getExtern(ip);
    }

    /// Always returns `null` for `status == .type_resolved`. This function is inteded
    /// to be used by code generation, since semantic analysis will ensure that any `Nav`
    /// which is potentially `extern` is fully resolved.
    /// Asserts that `status != .unresolved`.
    pub fn getExtern(nav: Nav, ip: *const InternPool) ?Key.Extern {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => null,
            .fully_resolved => |r| switch (ip.indexToKey(r.val)) {
                .@"extern" => |e| e,
                else => null,
            },
        };
    }

    /// Asserts that `status != .unresolved`.
    pub fn getAddrspace(nav: Nav) std.builtin.AddressSpace {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| r.@"addrspace",
            .fully_resolved => |r| r.@"addrspace",
        };
    }

    /// Asserts that `status != .unresolved`.
    pub fn getAlignment(nav: Nav) Alignment {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| r.alignment,
            .fully_resolved => |r| r.alignment,
        };
    }

    /// Asserts that `status != .unresolved`.
    pub fn getLinkSection(nav: Nav) OptionalNullTerminatedString {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| r.@"linksection",
            .fully_resolved => |r| r.@"linksection",
        };
    }

    /// Asserts that `status != .unresolved`.
    pub fn isThreadlocal(nav: Nav, ip: *const InternPool) bool {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| r.is_threadlocal,
            .fully_resolved => |r| switch (ip.indexToKey(r.val)) {
                .@"extern" => |e| e.is_threadlocal,
                .variable => |v| v.is_threadlocal,
                else => false,
            },
        };
    }

    pub fn isFn(nav: Nav, ip: *const InternPool) bool {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| {
                const tag = ip.zigTypeTag(r.type);
                return tag == .@"fn";
            },
            .fully_resolved => |r| {
                const tag = ip.zigTypeTag(ip.typeOf(r.val));
                return tag == .@"fn";
            },
        };
    }

    /// If this returns `true`, then a pointer to this `Nav` might actually be encoded as a pointer
    /// to some other `Nav` due to an extern definition or extern alias (see #21027).
    /// This query is valid on `Nav`s for whom only the type is resolved.
    /// Asserts that `status != .unresolved`.
    pub fn isExternOrFn(nav: Nav, ip: *const InternPool) bool {
        return switch (nav.status) {
            .unresolved => unreachable,
            .type_resolved => |r| {
                if (r.is_extern_decl) return true;
                const tag = ip.zigTypeTag(r.type);
                if (tag == .@"fn") return true;
                return false;
            },
            .fully_resolved => |r| {
                if (ip.indexToKey(r.val) == .@"extern") return true;
                const tag = ip.zigTypeTag(ip.typeOf(r.val));
                if (tag == .@"fn") return true;
                return false;
            },
        };
    }

    /// Get the ZIR instruction corresponding to this `Nav`, used to resolve source locations.
    /// This is a `declaration`.
    pub fn srcInst(nav: Nav, ip: *const InternPool) TrackedInst.Index {
        if (nav.analysis) |a| {
            return a.zir_index;
        }
        // A `Nav` which does not undergo analysis always has a resolved value.
        return switch (ip.indexToKey(nav.status.fully_resolved.val)) {
            .func => |func| {
                // Since `analysis` was not populated, this must be an instantiation.
                // Go up to the generic owner and consult *its* `analysis` field.
                const go_nav = ip.getNav(ip.indexToKey(func.generic_owner).func.owner_nav);
                return go_nav.analysis.?.zir_index;
            },
            .@"extern" => |@"extern"| @"extern".zir_index, // extern / @extern
            else => unreachable,
        };
    }

    pub const Index = enum(u32) {
        _,
        pub const Optional = enum(u32) {
            none = std.math.maxInt(u32),
            _,
            pub fn unwrap(opt: Optional) ?Nav.Index {
                return switch (opt) {
                    .none => null,
                    _ => @enumFromInt(@intFromEnum(opt)),
                };
            }

            const debug_state = InternPool.debug_state;
        };
        pub fn toOptional(i: Nav.Index) Optional {
            return @enumFromInt(@intFromEnum(i));
        }
        const Unwrapped = struct {
            tid: Zcu.PerThread.Id,
            index: u32,

            fn wrap(unwrapped: Unwrapped, ip: *const InternPool) Nav.Index {
                assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
                assert(unwrapped.index <= ip.getIndexMask(u32));
                return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                    unwrapped.index);
            }
        };
        fn unwrap(nav_index: Nav.Index, ip: *const InternPool) Unwrapped {
            return .{
                .tid = @enumFromInt(@intFromEnum(nav_index) >> ip.tid_shift_32 & ip.getTidMask()),
                .index = @intFromEnum(nav_index) & ip.getIndexMask(u32),
            };
        }

        const debug_state = InternPool.debug_state;
    };

    /// The compact in-memory representation of a `Nav`.
    /// 26 bytes.
    const Repr = struct {
        name: NullTerminatedString,
        fqn: NullTerminatedString,
        // The following 2 fields are either both populated, or both `.none`.
        analysis_namespace: OptionalNamespaceIndex,
        analysis_zir_index: TrackedInst.Index.Optional,
        /// Populated only if `bits.status != .unresolved`.
        type_or_val: InternPool.Index,
        /// Populated only if `bits.status != .unresolved`.
        @"linksection": OptionalNullTerminatedString,
        bits: Bits,

        const Bits = packed struct(u16) {
            status: enum(u2) { unresolved, type_resolved, fully_resolved, type_resolved_extern_decl },
            /// Populated only if `bits.status != .unresolved`.
            is_const: bool,
            /// Populated only if `bits.status != .unresolved`.
            alignment: Alignment,
            /// Populated only if `bits.status != .unresolved`.
            @"addrspace": std.builtin.AddressSpace,
            /// Populated only if `bits.status == .type_resolved`.
            is_threadlocal: bool,
            want_analysis: bool,
        };

        fn unpack(repr: Repr) Nav {
            return .{
                .name = repr.name,
                .fqn = repr.fqn,
                .analysis = if (repr.analysis_namespace.unwrap()) |namespace| .{
                    .namespace = namespace,
                    .zir_index = repr.analysis_zir_index.unwrap().?,
                    .wanted = repr.bits.want_analysis,
                } else a: {
                    assert(repr.analysis_zir_index == .none);
                    break :a null;
                },
                .status = switch (repr.bits.status) {
                    .unresolved => .unresolved,
                    .type_resolved, .type_resolved_extern_decl => .{ .type_resolved = .{
                        .type = repr.type_or_val,
                        .is_const = repr.bits.is_const,
                        .alignment = repr.bits.alignment,
                        .@"linksection" = repr.@"linksection",
                        .@"addrspace" = repr.bits.@"addrspace",
                        .is_threadlocal = repr.bits.is_threadlocal,
                        .is_extern_decl = repr.bits.status == .type_resolved_extern_decl,
                    } },
                    .fully_resolved => .{ .fully_resolved = .{
                        .val = repr.type_or_val,
                        .is_const = repr.bits.is_const,
                        .alignment = repr.bits.alignment,
                        .@"linksection" = repr.@"linksection",
                        .@"addrspace" = repr.bits.@"addrspace",
                    } },
                },
            };
        }
    };

    fn pack(nav: Nav) Repr {
        // Note that in the `unresolved` case, we do not mark fields as `undefined`, even though they should not be used.
        // This is to avoid writing undefined bytes to disk when serializing buffers.
        return .{
            .name = nav.name,
            .fqn = nav.fqn,
            .analysis_namespace = if (nav.analysis) |a| a.namespace.toOptional() else .none,
            .analysis_zir_index = if (nav.analysis) |a| a.zir_index.toOptional() else .none,
            .type_or_val = switch (nav.status) {
                .unresolved => .none,
                .type_resolved => |r| r.type,
                .fully_resolved => |r| r.val,
            },
            .@"linksection" = switch (nav.status) {
                .unresolved => .none,
                .type_resolved => |r| r.@"linksection",
                .fully_resolved => |r| r.@"linksection",
            },
            .bits = switch (nav.status) {
                .unresolved => .{
                    .status = .unresolved,
                    .is_const = false,
                    .alignment = .none,
                    .@"addrspace" = .generic,
                    .is_threadlocal = false,
                    .want_analysis = if (nav.analysis) |a| a.wanted else false,
                },
                .type_resolved => |r| .{
                    .status = if (r.is_extern_decl) .type_resolved_extern_decl else .type_resolved,
                    .is_const = r.is_const,
                    .alignment = r.alignment,
                    .@"addrspace" = r.@"addrspace",
                    .is_threadlocal = r.is_threadlocal,
                    .want_analysis = if (nav.analysis) |a| a.wanted else false,
                },
                .fully_resolved => |r| .{
                    .status = .fully_resolved,
                    .is_const = r.is_const,
                    .alignment = r.alignment,
                    .@"addrspace" = r.@"addrspace",
                    .is_threadlocal = false,
                    .want_analysis = if (nav.analysis) |a| a.wanted else false,
                },
            },
        };
    }
};

pub const Dependee = union(enum) {
    src_hash: TrackedInst.Index,
    nav_val: Nav.Index,
    nav_ty: Nav.Index,
    /// Index is the function, not its IES.
    func_ies: Index,
    type_layout: Index,
    zon_file: FileIndex,
    embed_file: Zcu.EmbedFile.Index,
    namespace: TrackedInst.Index,
    namespace_name: NamespaceNameKey,
    memoized_state: MemoizedStateStage,
};

pub fn removeDependenciesForDepender(ip: *InternPool, gpa: Allocator, depender: AnalUnit) void {
    var opt_idx = (ip.first_dependency.fetchSwapRemove(depender) orelse return).value.toOptional();

    while (opt_idx.unwrap()) |idx| {
        const dep = ip.dep_entries.items[@intFromEnum(idx)];
        opt_idx = dep.next_dependee;

        const prev_idx = dep.prev.unwrap() orelse {
            // This entry is the start of a list in some `*_deps`.
            // We cannot easily remove this mapping, so this must remain as a dummy entry.
            ip.dep_entries.items[@intFromEnum(idx)].depender = .none;
            continue;
        };

        ip.dep_entries.items[@intFromEnum(prev_idx)].next = dep.next;
        if (dep.next.unwrap()) |next_idx| {
            ip.dep_entries.items[@intFromEnum(next_idx)].prev = dep.prev;
        }

        ip.free_dep_entries.append(gpa, idx) catch {
            // This memory will be reclaimed on the next garbage collection.
            // Thus, we do not need to propagate this error.
        };
    }
}

pub const DependencyIterator = struct {
    ip: *const InternPool,
    next_entry: DepEntry.Index.Optional,
    pub fn next(it: *DependencyIterator) ?AnalUnit {
        while (true) {
            const idx = it.next_entry.unwrap() orelse return null;
            const entry = it.ip.dep_entries.items[@intFromEnum(idx)];
            it.next_entry = entry.next;
            if (entry.depender.unwrap()) |depender| return depender;
        }
    }
};

pub fn dependencyIterator(ip: *const InternPool, dependee: Dependee) DependencyIterator {
    const first_entry = switch (dependee) {
        .src_hash => |x| ip.src_hash_deps.get(x),
        .nav_val => |x| ip.nav_val_deps.get(x),
        .nav_ty => |x| ip.nav_ty_deps.get(x),
        .func_ies => |x| ip.func_ies_deps.get(x),
        .type_layout => |x| ip.type_layout_deps.get(x),
        .zon_file => |x| ip.zon_file_deps.get(x),
        .embed_file => |x| ip.embed_file_deps.get(x),
        .namespace => |x| ip.namespace_deps.get(x),
        .namespace_name => |x| ip.namespace_name_deps.get(x),
        .memoized_state => |stage| switch (stage) {
            .main => ip.memoized_state_main_deps.unwrap(),
            .panic => ip.memoized_state_panic_deps.unwrap(),
            .va_list => ip.memoized_state_va_list_deps.unwrap(),
            .assembly => ip.memoized_state_assembly_deps.unwrap(),
        },
    } orelse return .{
        .ip = ip,
        .next_entry = .none,
    };
    return .{
        .ip = ip,
        .next_entry = first_entry.toOptional(),
    };
}

pub fn addDependency(ip: *InternPool, gpa: Allocator, depender: AnalUnit, dependee: Dependee) Allocator.Error!void {
    const first_depender_dep: DepEntry.Index.Optional = if (ip.first_dependency.get(depender)) |idx| dep: {
        // The entry already exists, so there is capacity to overwrite it later.
        break :dep idx.toOptional();
    } else none: {
        // Ensure there is capacity available to add this dependency later.
        try ip.first_dependency.ensureUnusedCapacity(gpa, 1);
        break :none .none;
    };

    // We're very likely to need space for a new entry - reserve it now to avoid
    // the need for error cleanup logic.
    if (ip.free_dep_entries.items.len == 0) {
        try ip.dep_entries.ensureUnusedCapacity(gpa, 1);
    }

    // This block should allocate an entry and prepend it to the relevant `*_deps` list.
    // The `next` field should be correctly initialized; all other fields may be undefined.
    const new_index: DepEntry.Index = switch (dependee) {
        .memoized_state => |stage| new_index: {
            const deps = switch (stage) {
                .main => &ip.memoized_state_main_deps,
                .panic => &ip.memoized_state_panic_deps,
                .va_list => &ip.memoized_state_va_list_deps,
                .assembly => &ip.memoized_state_assembly_deps,
            };

            if (deps.unwrap()) |first| {
                if (ip.dep_entries.items[@intFromEnum(first)].depender == .none) {
                    // Dummy entry, so we can reuse it rather than allocating a new one!
                    break :new_index first;
                }
            }

            // Prepend a new dependency.
            const new_index: DepEntry.Index, const ptr = if (ip.free_dep_entries.pop()) |new_index| new: {
                break :new .{ new_index, &ip.dep_entries.items[@intFromEnum(new_index)] };
            } else .{ @enumFromInt(ip.dep_entries.items.len), ip.dep_entries.addOneAssumeCapacity() };
            if (deps.unwrap()) |old_first| {
                ptr.next = old_first.toOptional();
                ip.dep_entries.items[@intFromEnum(old_first)].prev = new_index.toOptional();
            } else {
                ptr.next = .none;
            }
            deps.* = new_index.toOptional();
            break :new_index new_index;
        },
        inline else => |dependee_payload, tag| new_index: {
            const gop = try switch (tag) {
                .src_hash => ip.src_hash_deps,
                .nav_val => ip.nav_val_deps,
                .nav_ty => ip.nav_ty_deps,
                .func_ies => ip.func_ies_deps,
                .type_layout => ip.type_layout_deps,
                .zon_file => ip.zon_file_deps,
                .embed_file => ip.embed_file_deps,
                .namespace => ip.namespace_deps,
                .namespace_name => ip.namespace_name_deps,
                .memoized_state => comptime unreachable,
            }.getOrPut(gpa, dependee_payload);

            if (gop.found_existing and ip.dep_entries.items[@intFromEnum(gop.value_ptr.*)].depender == .none) {
                // Dummy entry, so we can reuse it rather than allocating a new one!
                break :new_index gop.value_ptr.*;
            }

            // Prepend a new dependency.
            const new_index: DepEntry.Index, const ptr = if (ip.free_dep_entries.pop()) |new_index| new: {
                break :new .{ new_index, &ip.dep_entries.items[@intFromEnum(new_index)] };
            } else .{ @enumFromInt(ip.dep_entries.items.len), ip.dep_entries.addOneAssumeCapacity() };
            if (gop.found_existing) {
                ptr.next = gop.value_ptr.*.toOptional();
                ip.dep_entries.items[@intFromEnum(gop.value_ptr.*)].prev = new_index.toOptional();
            } else {
                ptr.next = .none;
            }
            gop.value_ptr.* = new_index;
            break :new_index new_index;
        },
    };

    ip.dep_entries.items[@intFromEnum(new_index)].depender = depender.toOptional();
    ip.dep_entries.items[@intFromEnum(new_index)].prev = .none;
    ip.dep_entries.items[@intFromEnum(new_index)].next_dependee = first_depender_dep;
    ip.first_dependency.putAssumeCapacity(depender, new_index);
}

/// String is the name whose existence the dependency is on.
/// DepEntry.Index refers to the first such dependency.
pub const NamespaceNameKey = struct {
    /// The instruction (`struct_decl` etc) which owns the namespace in question.
    namespace: TrackedInst.Index,
    /// The name whose existence the dependency is on.
    name: NullTerminatedString,
};

pub const DepEntry = extern struct {
    /// If null, this is a dummy entry. `next_dependee` is undefined. This is the first
    /// entry in one of `*_deps`, and does not appear in any list by `first_dependency`,
    /// but is not in `free_dep_entries` since `*_deps` stores a reference to it.
    depender: AnalUnit.Optional,
    /// Index into `dep_entries` forming a doubly linked list of all dependencies on this dependee.
    /// Used to iterate all dependers for a given dependee during an update.
    /// null if this is the end of the list.
    next: DepEntry.Index.Optional,
    /// The other link for `next`.
    /// null if this is the start of the list.
    prev: DepEntry.Index.Optional,
    /// Index into `dep_entries` forming a singly linked list of dependencies *of* `depender`.
    /// Used to efficiently remove all `DepEntry`s for a single `depender` when it is re-analyzed.
    /// null if this is the end of the list.
    next_dependee: DepEntry.Index.Optional,

    pub const Index = enum(u32) {
        _,
        pub fn toOptional(dep: DepEntry.Index) Optional {
            return @enumFromInt(@intFromEnum(dep));
        }
        pub const Optional = enum(u32) {
            none = std.math.maxInt(u32),
            _,
            pub fn unwrap(opt: Optional) ?DepEntry.Index {
                return switch (opt) {
                    .none => null,
                    _ => @enumFromInt(@intFromEnum(opt)),
                };
            }
        };
    };
};

const Local = struct {
    /// These fields can be accessed from any thread by calling `acquire`.
    /// They are only modified by the owning thread.
    shared: Shared align(std.atomic.cache_line),
    /// This state is fully local to the owning thread and does not require any
    /// atomic access.
    mutate: struct {
        /// When we need to allocate any long-lived buffer for mutating the `InternPool`, it is
        /// allocated into this `arena` (for the `Id` of the thread performing the mutation). An
        /// arena is used to avoid contention on the GPA, and to ensure that any code which retains
        /// references to old state remains valid. For instance, when reallocing hashmap metadata,
        /// a racing lookup on another thread may still retain a handle to the old metadata pointer,
        /// so it must remain valid.
        /// This arena's lifetime is tied to that of `Compilation`, although it can be cleared on
        /// garbage collection (currently vaporware).
        arena: std.heap.ArenaAllocator.State,

        items: ListMutate,
        extra: ListMutate,
        limbs: ListMutate,
        strings: ListMutate,
        string_bytes: ListMutate,
        tracked_insts: ListMutate,
        files: ListMutate,
        maps: ListMutate,
        navs: ListMutate,
        comptime_units: ListMutate,

        namespaces: BucketListMutate,
    } align(std.atomic.cache_line),

    const Shared = struct {
        items: List(Item),
        extra: Extra,
        limbs: Limbs,
        strings: Strings,
        string_bytes: StringBytes,
        tracked_insts: TrackedInsts,
        files: List(File),
        maps: Maps,
        navs: Navs,
        comptime_units: ComptimeUnits,

        namespaces: Namespaces,

        pub fn getLimbs(shared: *const Local.Shared) Limbs {
            return switch (@sizeOf(Limb)) {
                @sizeOf(u32) => shared.extra,
                @sizeOf(u64) => shared.limbs,
                else => @compileError("unsupported host"),
            }.acquire();
        }
    };

    const Extra = List(struct { u32 });
    const Limbs = switch (@sizeOf(Limb)) {
        @sizeOf(u32) => Extra,
        @sizeOf(u64) => List(struct { u64 }),
        else => @compileError("unsupported host"),
    };
    const Strings = List(struct { u32 });
    const StringBytes = List(struct { u8 });
    const TrackedInsts = List(struct { TrackedInst.MaybeLost });
    const Maps = List(struct { FieldMap });
    const Navs = List(Nav.Repr);
    const ComptimeUnits = List(struct { ComptimeUnit });

    const namespaces_bucket_width = 8;
    const namespaces_bucket_mask = (1 << namespaces_bucket_width) - 1;
    const namespace_next_free_field = "owner_type";
    const Namespaces = List(struct { *[1 << namespaces_bucket_width]Zcu.Namespace });

    const ListMutate = struct {
        mutex: Io.Mutex,
        len: u32,

        const empty: ListMutate = .{
            .mutex = .init,
            .len = 0,
        };
    };

    const BucketListMutate = struct {
        last_bucket_len: u32,
        buckets_list: ListMutate,
        free_list: u32,

        const free_list_sentinel = std.math.maxInt(u32);

        const empty: BucketListMutate = .{
            .last_bucket_len = 0,
            .buckets_list = ListMutate.empty,
            .free_list = free_list_sentinel,
        };
    };

    fn List(comptime Elem: type) type {
        assert(@typeInfo(Elem) == .@"struct");
        return struct {
            bytes: [*]align(@alignOf(Elem)) u8,

            const ListSelf = @This();
            const Mutable = struct {
                gpa: Allocator,
                io: Io,
                arena: *std.heap.ArenaAllocator.State,
                mutate: *ListMutate,
                list: *ListSelf,

                const fields = std.enums.values(std.meta.FieldEnum(Elem));

                fn PtrArrayElem(comptime len: usize) type {
                    const elem_info = @typeInfo(Elem).@"struct";
                    const elem_fields = elem_info.fields;
                    var new_names: [elem_fields.len][]const u8 = undefined;
                    var new_types: [elem_fields.len]type = undefined;
                    for (elem_fields, &new_names, &new_types) |elem_field, *new_name, *NewType| {
                        new_name.* = elem_field.name;
                        NewType.* = *[len]elem_field.type;
                    }
                    if (elem_info.is_tuple) {
                        return @Tuple(&new_types);
                    } else {
                        return @Struct(.auto, null, &new_names, &new_types, &@splat(.{}));
                    }
                }
                fn PtrElem(comptime opts: struct {
                    size: std.builtin.Type.Pointer.Size,
                    is_const: bool = false,
                }) type {
                    const elem_info = @typeInfo(Elem).@"struct";
                    const elem_fields = elem_info.fields;
                    var new_names: [elem_fields.len][]const u8 = undefined;
                    var new_types: [elem_fields.len]type = undefined;
                    for (elem_fields, &new_names, &new_types) |elem_field, *new_name, *NewType| {
                        new_name.* = elem_field.name;
                        NewType.* = @Pointer(opts.size, .{ .@"const" = opts.is_const }, elem_field.type, null);
                    }
                    if (elem_info.is_tuple) {
                        return @Tuple(&new_types);
                    } else {
                        return @Struct(.auto, null, &new_names, &new_types, &@splat(.{}));
                    }
                }

                pub fn addOne(mutable: Mutable) Allocator.Error!PtrElem(.{ .size = .one }) {
                    try mutable.ensureUnusedCapacity(1);
                    return mutable.addOneAssumeCapacity();
                }

                pub fn addOneAssumeCapacity(mutable: Mutable) PtrElem(.{ .size = .one }) {
                    const index = mutable.mutate.len;
                    assert(index < mutable.list.header().capacity);
                    mutable.mutate.len = index + 1;
                    const mutable_view = mutable.view().slice();
                    var ptr: PtrElem(.{ .size = .one }) = undefined;
                    inline for (fields) |field| {
                        @field(ptr, @tagName(field)) = &mutable_view.items(field)[index];
                    }
                    return ptr;
                }

                pub fn append(mutable: Mutable, elem: Elem) Allocator.Error!void {
                    try mutable.ensureUnusedCapacity(1);
                    mutable.appendAssumeCapacity(elem);
                }

                pub fn appendAssumeCapacity(mutable: Mutable, elem: Elem) void {
                    var mutable_view = mutable.view();
                    defer mutable.mutate.len = @intCast(mutable_view.len);
                    mutable_view.appendAssumeCapacity(elem);
                }

                pub fn appendSliceAssumeCapacity(
                    mutable: Mutable,
                    slice: PtrElem(.{ .size = .slice, .is_const = true }),
                ) void {
                    if (fields.len == 0) return;
                    const start = mutable.mutate.len;
                    const slice_len = @field(slice, @tagName(fields[0])).len;
                    assert(slice_len <= mutable.list.header().capacity - start);
                    mutable.mutate.len = @intCast(start + slice_len);
                    const mutable_view = mutable.view().slice();
                    inline for (fields) |field| {
                        const field_slice = @field(slice, @tagName(field));
                        assert(field_slice.len == slice_len);
                        @memcpy(mutable_view.items(field)[start..][0..slice_len], field_slice);
                    }
                }

                pub fn appendNTimes(mutable: Mutable, elem: Elem, len: usize) Allocator.Error!void {
                    try mutable.ensureUnusedCapacity(len);
                    mutable.appendNTimesAssumeCapacity(elem, len);
                }

                pub fn appendNTimesAssumeCapacity(mutable: Mutable, elem: Elem, len: usize) void {
                    const start = mutable.mutate.len;
                    assert(len <= mutable.list.header().capacity - start);
                    mutable.mutate.len = @intCast(start + len);
                    const mutable_view = mutable.view().slice();
                    inline for (fields) |field| {
                        @memset(mutable_view.items(field)[start..][0..len], @field(elem, @tagName(field)));
                    }
                }

                pub fn addManyAsArray(mutable: Mutable, comptime len: usize) Allocator.Error!PtrArrayElem(len) {
                    try mutable.ensureUnusedCapacity(len);
                    return mutable.addManyAsArrayAssumeCapacity(len);
                }

                pub fn addManyAsArrayAssumeCapacity(mutable: Mutable, comptime len: usize) PtrArrayElem(len) {
                    const start = mutable.mutate.len;
                    assert(len <= mutable.list.header().capacity - start);
                    mutable.mutate.len = @intCast(start + len);
                    const mutable_view = mutable.view().slice();
                    var ptr_array: PtrArrayElem(len) = undefined;
                    inline for (fields) |field| {
                        @field(ptr_array, @tagName(field)) = mutable_view.items(field)[start..][0..len];
                    }
                    return ptr_array;
                }

                pub fn addManyAsSlice(mutable: Mutable, len: usize) Allocator.Error!PtrElem(.{ .size = .slice }) {
                    try mutable.ensureUnusedCapacity(len);
                    return mutable.addManyAsSliceAssumeCapacity(len);
                }

                pub fn addManyAsSliceAssumeCapacity(mutable: Mutable, len: usize) PtrElem(.{ .size = .slice }) {
                    const start = mutable.mutate.len;
                    assert(len <= mutable.list.header().capacity - start);
                    mutable.mutate.len = @intCast(start + len);
                    const mutable_view = mutable.view().slice();
                    var slice: PtrElem(.{ .size = .slice }) = undefined;
                    inline for (fields) |field| {
                        @field(slice, @tagName(field)) = mutable_view.items(field)[start..][0..len];
                    }
                    return slice;
                }

                pub fn shrinkRetainingCapacity(mutable: Mutable, len: usize) void {
                    assert(len <= mutable.mutate.len);
                    mutable.mutate.len = @intCast(len);
                }

                pub fn ensureUnusedCapacity(mutable: Mutable, unused_capacity: usize) Allocator.Error!void {
                    try mutable.ensureTotalCapacity(@intCast(mutable.mutate.len + unused_capacity));
                }

                pub fn ensureTotalCapacity(mutable: Mutable, total_capacity: usize) Allocator.Error!void {
                    const old_capacity = mutable.list.header().capacity;
                    if (old_capacity >= total_capacity) return;
                    var new_capacity = old_capacity;
                    while (new_capacity < total_capacity) new_capacity = (new_capacity + 10) * 2;
                    try mutable.setCapacity(new_capacity);
                }

                fn setCapacity(mutable: Mutable, capacity: u32) Allocator.Error!void {
                    const io = mutable.io;
                    var arena = mutable.arena.promote(mutable.gpa);
                    defer mutable.arena.* = arena.state;
                    const buf = try arena.allocator().alignedAlloc(
                        u8,
                        .fromByteUnits(alignment),
                        bytes_offset + View.capacityInBytes(capacity),
                    );
                    var new_list: ListSelf = .{ .bytes = @ptrCast(buf[bytes_offset..].ptr) };
                    new_list.header().* = .{ .capacity = capacity };
                    const len = mutable.mutate.len;
                    // this cold, quickly predictable, condition enables
                    // the `MultiArrayList` optimization in `view`
                    if (len > 0) {
                        const old_slice = mutable.list.view().slice();
                        const new_slice = new_list.view().slice();
                        inline for (fields) |field| @memcpy(new_slice.items(field)[0..len], old_slice.items(field)[0..len]);
                    }
                    mutable.mutate.mutex.lockUncancelable(io);
                    defer mutable.mutate.mutex.unlock(io);
                    mutable.list.release(new_list);
                }

                pub fn viewAllowEmpty(mutable: Mutable) View {
                    const capacity = mutable.list.header().capacity;
                    return .{
                        .bytes = mutable.list.bytes,
                        .len = mutable.mutate.len,
                        .capacity = capacity,
                    };
                }
                pub fn view(mutable: Mutable) View {
                    const capacity = mutable.list.header().capacity;
                    assert(capacity > 0); // optimizes `MultiArrayList.Slice.items`
                    return .{
                        .bytes = mutable.list.bytes,
                        .len = mutable.mutate.len,
                        .capacity = capacity,
                    };
                }
            };

            const empty: ListSelf = .{ .bytes = @constCast(&(extern struct {
                header: Header,
                bytes: [0]u8 align(@alignOf(Elem)),
            }{
                .header = .{ .capacity = 0 },
                .bytes = .{},
            }).bytes) };

            const alignment = @max(@alignOf(Header), @alignOf(Elem));
            const bytes_offset = std.mem.alignForward(usize, @sizeOf(Header), @alignOf(Elem));
            const View = std.MultiArrayList(Elem);

            /// Must be called when accessing from another thread.
            pub fn acquire(list: *const ListSelf) ListSelf {
                return .{ .bytes = @atomicLoad([*]align(@alignOf(Elem)) u8, &list.bytes, .acquire) };
            }
            fn release(list: *ListSelf, new_list: ListSelf) void {
                @atomicStore([*]align(@alignOf(Elem)) u8, &list.bytes, new_list.bytes, .release);
            }

            const Header = extern struct {
                capacity: u32,
            };
            fn header(list: ListSelf) *Header {
                return @ptrCast(@alignCast(list.bytes - bytes_offset));
            }
            pub fn view(list: ListSelf) View {
                const capacity = list.header().capacity;
                assert(capacity > 0); // optimizes `MultiArrayList.Slice.items`
                return .{
                    .bytes = list.bytes,
                    .len = capacity,
                    .capacity = capacity,
                };
            }
        };
    }

    pub fn getMutableItems(local: *Local, gpa: Allocator, io: Io) List(Item).Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.items,
            .list = &local.shared.items,
        };
    }

    pub fn getMutableExtra(local: *Local, gpa: Allocator, io: Io) Extra.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.extra,
            .list = &local.shared.extra,
        };
    }

    /// On 32-bit systems, this array is ignored and extra is used for everything.
    /// On 64-bit systems, this array is used for big integers and associated metadata.
    /// Use the helper methods instead of accessing this directly in order to not
    /// violate the above mechanism.
    pub fn getMutableLimbs(local: *Local, gpa: Allocator, io: Io) Limbs.Mutable {
        return switch (@sizeOf(Limb)) {
            @sizeOf(u32) => local.getMutableExtra(gpa, io),
            @sizeOf(u64) => .{
                .gpa = gpa,
                .io = io,
                .arena = &local.mutate.arena,
                .mutate = &local.mutate.limbs,
                .list = &local.shared.limbs,
            },
            else => @compileError("unsupported host"),
        };
    }

    /// A list of offsets into `string_bytes` for each string.
    pub fn getMutableStrings(local: *Local, gpa: Allocator, io: Io) Strings.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.strings,
            .list = &local.shared.strings,
        };
    }

    /// In order to store references to strings in fewer bytes, we copy all
    /// string bytes into here. String bytes can be null. It is up to whomever
    /// is referencing the data here whether they want to store both index and length,
    /// thus allowing null bytes, or store only index, and use null-termination. The
    /// `strings_bytes` array is agnostic to either usage.
    pub fn getMutableStringBytes(local: *Local, gpa: Allocator, io: Io) StringBytes.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.string_bytes,
            .list = &local.shared.string_bytes,
        };
    }

    /// An index into `tracked_insts` gives a reference to a single ZIR instruction which
    /// persists across incremental updates.
    pub fn getMutableTrackedInsts(local: *Local, gpa: Allocator, io: Io) TrackedInsts.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.tracked_insts,
            .list = &local.shared.tracked_insts,
        };
    }

    /// Elements are ordered identically to the `import_table` field of `Zcu`.
    ///
    /// Unlike `import_table`, this data is serialized as part of incremental
    /// compilation state.
    ///
    /// Key is the hash of the path to this file, used to store
    /// `InternPool.TrackedInst`.
    pub fn getMutableFiles(local: *Local, gpa: Allocator, io: Io) List(File).Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.files,
            .list = &local.shared.files,
        };
    }

    /// Some types such as enums, structs, and unions need to store mappings from field names
    /// to field index, or value to field index. In such cases, they will store the underlying
    /// field names and values directly, relying on one of these maps, stored separately,
    /// to provide lookup.
    /// These are not serialized; it is computed upon deserialization.
    pub fn getMutableMaps(local: *Local, gpa: Allocator, io: Io) Maps.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.maps,
            .list = &local.shared.maps,
        };
    }

    pub fn getMutableNavs(local: *Local, gpa: Allocator, io: Io) Navs.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.navs,
            .list = &local.shared.navs,
        };
    }

    pub fn getMutableComptimeUnits(local: *Local, gpa: Allocator, io: Io) ComptimeUnits.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.comptime_units,
            .list = &local.shared.comptime_units,
        };
    }

    /// Rather than allocating Namespace objects with an Allocator, we instead allocate
    /// them with this BucketList. This provides four advantages:
    ///  * Stable memory so that one thread can access a Namespace object while another
    ///    thread allocates additional Namespace objects from this list.
    ///  * It allows us to use u32 indexes to reference Namespace objects rather than
    ///    pointers, saving memory in types.
    ///  * Using integers to reference Namespace objects rather than pointers makes
    ///    serialization trivial.
    ///  * It provides a unique integer to be used for anonymous symbol names, avoiding
    ///    multi-threaded contention on an atomic counter.
    pub fn getMutableNamespaces(local: *Local, gpa: Allocator, io: Io) Namespaces.Mutable {
        return .{
            .gpa = gpa,
            .io = io,
            .arena = &local.mutate.arena,
            .mutate = &local.mutate.namespaces.buckets_list,
            .list = &local.shared.namespaces,
        };
    }
};

pub fn getLocal(ip: *InternPool, tid: Zcu.PerThread.Id) *Local {
    return &ip.locals[@intFromEnum(tid)];
}

pub fn getLocalShared(ip: *const InternPool, tid: Zcu.PerThread.Id) *const Local.Shared {
    return &ip.locals[@intFromEnum(tid)].shared;
}

const Shard = struct {
    shared: struct {
        map: Map(Index),
        string_map: Map(OptionalNullTerminatedString),
        tracked_inst_map: Map(TrackedInst.Index.Optional),
    } align(std.atomic.cache_line),
    mutate: struct {
        // TODO: measure cost of sharing unrelated mutate state
        map: Mutate align(std.atomic.cache_line),
        string_map: Mutate align(std.atomic.cache_line),
        tracked_inst_map: Mutate align(std.atomic.cache_line),
    },

    const Mutate = struct {
        /// This mutex needs to be recursive because `getFuncDeclIes` interns multiple things at
        /// once (the function, its IES, the corresponding error union, and the resulting function
        /// type), so calls `getOrPutKeyEnsuringAdditionalCapacity` multiple times. Each of these
        /// calls acquires a lock which will only be released when the whole operation is finalized,
        /// and these different items could be in the same shard, in which case that shard's lock
        /// will be acquired multiple times.
        mutex: RecursiveMutex,
        len: u32,

        const RecursiveMutex = struct {
            const OptionalTid = if (single_threaded) enum(u8) {
                null,
                main,
                fn unwrap(ot: OptionalTid) ?Zcu.PerThread.Id {
                    return switch (ot) {
                        .null => null,
                        .main => .main,
                    };
                }
                fn wrap(tid: Zcu.PerThread.Id) OptionalTid {
                    comptime assert(tid == .main);
                    return .main;
                }
            } else packed struct(u8) {
                non_null: bool,
                value: Zcu.PerThread.Id,
                const @"null": OptionalTid = .{ .non_null = false, .value = .main };
                fn unwrap(ot: OptionalTid) ?Zcu.PerThread.Id {
                    return if (ot.non_null) ot.value else null;
                }
                fn wrap(tid: Zcu.PerThread.Id) OptionalTid {
                    return .{ .non_null = true, .value = tid };
                }
            };
            mutex: Io.Mutex,
            tid: std.atomic.Value(OptionalTid),
            lock_count: u32,
            const init: RecursiveMutex = .{ .mutex = .init, .tid = .init(.null), .lock_count = 0 };
            fn lock(r: *RecursiveMutex, io: Io, tid: Zcu.PerThread.Id) void {
                if (r.tid.load(.monotonic) != OptionalTid.wrap(tid)) {
                    r.mutex.lockUncancelable(io);
                    assert(r.lock_count == 0);
                    r.tid.store(.wrap(tid), .monotonic);
                }
                r.lock_count += 1;
            }
            fn unlock(r: *RecursiveMutex, io: Io) void {
                r.lock_count -= 1;
                if (r.lock_count == 0) {
                    r.tid.store(.null, .monotonic);
                    r.mutex.unlock(io);
                }
            }
        };

        const empty: Mutate = .{
            .mutex = .init,
            .len = 0,
        };
    };

    fn Map(comptime Value: type) type {
        comptime assert(@typeInfo(Value).@"enum".tag_type == u32);
        _ = @as(Value, .none); // expected .none key
        return struct {
            /// header: Header,
            /// entries: [header.capacity]Entry,
            entries: [*]Entry,

            const empty: @This() = .{ .entries = @constCast(&(extern struct {
                header: Header,
                entries: [1]Entry,
            }{
                .header = .{ .capacity = 1 },
                .entries = .{.{ .value = .none, .hash = undefined }},
            }).entries) };

            const alignment = @max(@alignOf(Header), @alignOf(Entry));
            const entries_offset = std.mem.alignForward(usize, @sizeOf(Header), @alignOf(Entry));

            /// Must be called unless the mutate mutex is locked.
            fn acquire(map: *const @This()) @This() {
                return .{ .entries = @atomicLoad([*]Entry, &map.entries, .acquire) };
            }
            fn release(map: *@This(), new_map: @This()) void {
                @atomicStore([*]Entry, &map.entries, new_map.entries, .release);
            }

            const Header = extern struct {
                capacity: u32,

                fn mask(head: *const Header) u32 {
                    assert(std.math.isPowerOfTwo(head.capacity));
                    return head.capacity - 1;
                }
            };
            fn header(map: @This()) *Header {
                return @ptrCast(@alignCast(@as([*]u8, @ptrCast(map.entries)) - entries_offset));
            }

            const Entry = extern struct {
                value: Value,
                hash: u32,

                fn acquire(entry: *const Entry) Value {
                    return @atomicLoad(Value, &entry.value, .acquire);
                }
                fn release(entry: *Entry, value: Value) void {
                    assert(value != .none);
                    @atomicStore(Value, &entry.value, value, .release);
                }
                fn resetUnordered(entry: *Entry) void {
                    @atomicStore(Value, &entry.value, .none, .unordered);
                }
            };
        };
    }
};

fn getTidMask(ip: *const InternPool) u32 {
    return @shlExact(@as(u32, 1), ip.tid_width) - 1;
}

fn getIndexMask(ip: *const InternPool, comptime BackingInt: type) u32 {
    return @as(u32, std.math.maxInt(BackingInt)) >> ip.tid_width;
}

const FieldMap = std.ArrayHashMapUnmanaged(void, void, std.array_hash_map.AutoContext(void), false);

/// An index into `maps` which might be `none`.
pub const OptionalMapIndex = enum(u32) {
    none = std.math.maxInt(u32),
    _,

    pub fn unwrap(oi: OptionalMapIndex) ?MapIndex {
        if (oi == .none) return null;
        return @enumFromInt(@intFromEnum(oi));
    }
};

/// An index into `maps`.
pub const MapIndex = enum(u32) {
    _,

    pub fn get(map_index: MapIndex, ip: *const InternPool) *FieldMap {
        const unwrapped_map_index = map_index.unwrap(ip);
        const maps = ip.getLocalShared(unwrapped_map_index.tid).maps.acquire();
        return &maps.view().items(.@"0")[unwrapped_map_index.index];
    }

    pub fn toOptional(i: MapIndex) OptionalMapIndex {
        return @enumFromInt(@intFromEnum(i));
    }

    const Unwrapped = struct {
        tid: Zcu.PerThread.Id,
        index: u32,

        fn wrap(unwrapped: Unwrapped, ip: *const InternPool) MapIndex {
            assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
            assert(unwrapped.index <= ip.getIndexMask(u32));
            return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                unwrapped.index);
        }
    };
    fn unwrap(map_index: MapIndex, ip: *const InternPool) Unwrapped {
        return .{
            .tid = @enumFromInt(@intFromEnum(map_index) >> ip.tid_shift_32 & ip.getTidMask()),
            .index = @intFromEnum(map_index) & ip.getIndexMask(u32),
        };
    }
};

pub const ComptimeAllocIndex = enum(u32) { _ };

pub const NamespaceIndex = enum(u32) {
    _,

    const Unwrapped = struct {
        tid: Zcu.PerThread.Id,
        bucket_index: u32,
        index: u32,

        fn wrap(unwrapped: Unwrapped, ip: *const InternPool) NamespaceIndex {
            assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
            assert(unwrapped.bucket_index <= ip.getIndexMask(u32) >> Local.namespaces_bucket_width);
            assert(unwrapped.index <= Local.namespaces_bucket_mask);
            return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                unwrapped.bucket_index << Local.namespaces_bucket_width |
                unwrapped.index);
        }
    };
    fn unwrap(namespace_index: NamespaceIndex, ip: *const InternPool) Unwrapped {
        const index = @intFromEnum(namespace_index) & ip.getIndexMask(u32);
        return .{
            .tid = @enumFromInt(@intFromEnum(namespace_index) >> ip.tid_shift_32 & ip.getTidMask()),
            .bucket_index = index >> Local.namespaces_bucket_width,
            .index = index & Local.namespaces_bucket_mask,
        };
    }

    pub fn toOptional(i: NamespaceIndex) OptionalNamespaceIndex {
        return @enumFromInt(@intFromEnum(i));
    }
};

pub const OptionalNamespaceIndex = enum(u32) {
    none = std.math.maxInt(u32),
    _,

    pub fn init(oi: ?NamespaceIndex) OptionalNamespaceIndex {
        return @enumFromInt(@intFromEnum(oi orelse return .none));
    }

    pub fn unwrap(oi: OptionalNamespaceIndex) ?NamespaceIndex {
        if (oi == .none) return null;
        return @enumFromInt(@intFromEnum(oi));
    }
};

pub const FileIndex = enum(u32) {
    _,

    const Unwrapped = struct {
        tid: Zcu.PerThread.Id,
        index: u32,

        fn wrap(unwrapped: Unwrapped, ip: *const InternPool) FileIndex {
            assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
            assert(unwrapped.index <= ip.getIndexMask(u32));
            return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                unwrapped.index);
        }
    };
    pub fn unwrap(file_index: FileIndex, ip: *const InternPool) Unwrapped {
        return .{
            .tid = @enumFromInt(@intFromEnum(file_index) >> ip.tid_shift_32 & ip.getTidMask()),
            .index = @intFromEnum(file_index) & ip.getIndexMask(u32),
        };
    }
    pub fn toOptional(i: FileIndex) Optional {
        return @enumFromInt(@intFromEnum(i));
    }
    pub const Optional = enum(u32) {
        none = std.math.maxInt(u32),
        _,
        pub fn unwrap(opt: Optional) ?FileIndex {
            return switch (opt) {
                .none => null,
                _ => @enumFromInt(@intFromEnum(opt)),
            };
        }
    };
};

const File = struct {
    bin_digest: Cache.BinDigest,
    file: *Zcu.File,
    /// `.none` means no type has been created yet.
    root_type: InternPool.Index,
};

/// An index into `strings`.
pub const String = enum(u32) {
    /// An empty string.
    empty = 0,
    _,

    pub fn toSlice(string: String, len: u64, ip: *const InternPool) []const u8 {
        return string.toOverlongSlice(ip)[0..@intCast(len)];
    }

    pub fn at(string: String, index: u64, ip: *const InternPool) u8 {
        return string.toOverlongSlice(ip)[@intCast(index)];
    }

    pub fn toNullTerminatedString(string: String, len: u64, ip: *const InternPool) NullTerminatedString {
        assert(std.mem.indexOfScalar(u8, string.toSlice(len, ip), 0) == null);
        assert(string.at(len, ip) == 0);
        return @enumFromInt(@intFromEnum(string));
    }

    const Unwrapped = struct {
        tid: Zcu.PerThread.Id,
        index: u32,

        fn wrap(unwrapped: Unwrapped, ip: *const InternPool) String {
            assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
            assert(unwrapped.index <= ip.getIndexMask(u32));
            return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_32) |
                unwrapped.index);
        }
    };
    fn unwrap(string: String, ip: *const InternPool) Unwrapped {
        return .{
            .tid = @enumFromInt(@intFromEnum(string) >> ip.tid_shift_32 & ip.getTidMask()),
            .index = @intFromEnum(string) & ip.getIndexMask(u32),
        };
    }

    fn toOverlongSlice(string: String, ip: *const InternPool) []const u8 {
        const unwrapped = string.unwrap(ip);
        const local_shared = ip.getLocalShared(unwrapped.tid);
        const strings = local_shared.strings.acquire().view().items(.@"0");
        const string_bytes = local_shared.string_bytes.acquire().view().items(.@"0");
        return string_bytes[strings[unwrapped.index]..];
    }

    const debug_state = InternPool.debug_state;
};

/// An index into `strings` which might be `none`.
pub const OptionalString = enum(u32) {
    /// This is distinct from `none` - it is a valid index that represents empty string.
    empty = 0,
    none = std.math.maxInt(u32),
    _,

    pub fn unwrap(string: OptionalString) ?String {
        return if (string != .none) @enumFromInt(@intFromEnum(string)) else null;
    }

    pub fn toSlice(string: OptionalString, len: u64, ip: *const InternPool) ?[]const u8 {
        return (string.unwrap() orelse return null).toSlice(len, ip);
    }

    const debug_state = InternPool.debug_state;
};

/// An index into `strings`.
pub const NullTerminatedString = enum(u32) {
    /// An empty string.
    empty = 0,
    _,

    /// An array of `NullTerminatedString` existing within the `extra` array.
    /// This type exists to provide a struct with lifetime that is
    /// not invalidated when items are added to the `InternPool`.
    pub const Slice = struct {
        tid: Zcu.PerThread.Id,
        start: u32,
        len: u32,

        pub const empty: Slice = .{ .tid = .main, .start = 0, .len = 0 };

        pub fn get(slice: Slice, ip: *const InternPool) []NullTerminatedString {
            const extra = ip.getLocalShared(slice.tid).extra.acquire();
            return @ptrCast(extra.view().items(.@"0")[slice.start..][0..slice.len]);
        }
    };

    pub fn toString(self: NullTerminatedString) String {
        return @enumFromInt(@intFromEnum(self));
    }

    pub fn toOptional(self: NullTerminatedString) OptionalNullTerminatedString {
        return @enumFromInt(@intFromEnum(self));
    }

    pub fn toSlice(string: NullTerminatedString, ip: *const InternPool) [:0]const u8 {
        const unwrapped = string.toString().unwrap(ip);
        const local_shared = ip.getLocalShared(unwrapped.tid);
        const strings = local_shared.strings.acquire().view().items(.@"0");
        const string_bytes = local_shared.string_bytes.acquire().view().items(.@"0");
        return string_bytes[strings[unwrapped.index] .. strings[unwrapped.index + 1] - 1 :0];
    }

    pub fn length(string: NullTerminatedString, ip: *const InternPool) u32 {
        const unwrapped = string.toString().unwrap(ip);
        const local_shared = ip.getLocalShared(unwrapped.tid);
        const strings = local_shared.strings.acquire().view().items(.@"0");
        return strings[unwrapped.index + 1] - 1 - strings[unwrapped.index];
    }

    pub fn eqlSlice(string: NullTerminatedString, slice: []const u8, ip: *const InternPool) bool {
        const overlong_slice = string.toString().toOverlongSlice(ip);
        return overlong_slice.len > slice.len and
            std.mem.eql(u8, overlong_slice[0..slice.len], slice) and
            overlong_slice[slice.len] == 0;
    }

    const Adapter = struct {
        strings: []const NullTerminatedString,

        pub fn eql(ctx: @This(), a: NullTerminatedString, b_void: void, b_map_index: usize) bool {
            _ = b_void;
            return a == ctx.strings[b_map_index];
        }

        pub fn hash(ctx: @This(), a: NullTerminatedString) u32 {
            _ = ctx;
            return std.hash.int(@intFromEnum(a));
        }
    };

    /// Compare based on integer value alone, ignoring the string contents.
    pub fn indexLessThan(ctx: void, a: NullTerminatedString, b: NullTerminatedString) bool {
        _ = ctx;
        return @intFromEnum(a) < @intFromEnum(b);
    }

    pub fn toUnsigned(string: NullTerminatedString, ip: *const InternPool) ?u32 {
        const slice = string.toSlice(ip);
        if (slice.len > 1 and slice[0] == '0') return null;
        if (std.mem.indexOfScalar(u8, slice, '_')) |_| return null;
        return std.fmt.parseUnsigned(u32, slice, 10) catch null;
    }

    const FormatData = struct {
        string: NullTerminatedString,
        ip: *const InternPool,
        id: bool,
    };
    fn format(data: FormatData, writer: *Io.Writer) Io.Writer.Error!void {
        const slice = data.string.toSlice(data.ip);
        if (!data.id) {
            try writer.writeAll(slice);
        } else {
            try writer.print("{f}", .{std.zig.fmtIdP(slice)});
        }
    }

    pub fn fmt(string: NullTerminatedString, ip: *const InternPool) std.fmt.Alt(FormatData, format) {
        return .{ .data = .{ .string = string, .ip = ip, .id = false } };
    }

    pub fn fmtId(string: NullTerminatedString, ip: *const InternPool) std.fmt.Alt(FormatData, format) {
        return .{ .data = .{ .string = string, .ip = ip, .id = true } };
    }

    const debug_state = InternPool.debug_state;
};

/// An index into `strings` which might be `none`.
pub const OptionalNullTerminatedString = enum(u32) {
    /// This is distinct from `none` - it is a valid index that represents empty string.
    empty = 0,
    none = std.math.maxInt(u32),
    _,

    pub fn unwrap(string: OptionalNullTerminatedString) ?NullTerminatedString {
        return if (string != .none) @enumFromInt(@intFromEnum(string)) else null;
    }

    pub fn toSlice(string: OptionalNullTerminatedString, ip: *const InternPool) ?[:0]const u8 {
        return (string.unwrap() orelse return null).toSlice(ip);
    }

    const debug_state = InternPool.debug_state;
};

/// A single value captured in the closure of a namespace type. This is not a plain
/// `Index` because we must differentiate between the following cases:
/// * runtime-known value (where we store the type)
/// * comptime-known value (where we store the value)
/// * `Nav` val (so that we can analyze the value lazily)
/// * `Nav` ref (so that we can analyze the reference lazily)
pub const CaptureValue = packed struct(u32) {
    tag: enum(u2) { @"comptime", runtime, nav_val, nav_ref },
    idx: u30,

    pub fn wrap(val: Unwrapped) CaptureValue {
        return switch (val) {
            .@"comptime" => |i| .{ .tag = .@"comptime", .idx = @intCast(@intFromEnum(i)) },
            .runtime => |i| .{ .tag = .runtime, .idx = @intCast(@intFromEnum(i)) },
            .nav_val => |i| .{ .tag = .nav_val, .idx = @intCast(@intFromEnum(i)) },
            .nav_ref => |i| .{ .tag = .nav_ref, .idx = @intCast(@intFromEnum(i)) },
        };
    }
    pub fn unwrap(val: CaptureValue) Unwrapped {
        return switch (val.tag) {
            .@"comptime" => .{ .@"comptime" = @enumFromInt(val.idx) },
            .runtime => .{ .runtime = @enumFromInt(val.idx) },
            .nav_val => .{ .nav_val = @enumFromInt(val.idx) },
            .nav_ref => .{ .nav_ref = @enumFromInt(val.idx) },
        };
    }

    pub const Unwrapped = union(enum) {
        /// Index refers to the value.
        @"comptime": Index,
        /// Index refers to the type.
        runtime: Index,
        nav_val: Nav.Index,
        nav_ref: Nav.Index,
    };

    pub const Slice = struct {
        tid: Zcu.PerThread.Id,
        start: u32,
        len: u32,

        pub const empty: Slice = .{ .tid = .main, .start = 0, .len = 0 };

        pub fn get(slice: Slice, ip: *const InternPool) []CaptureValue {
            const extra = ip.getLocalShared(slice.tid).extra.acquire();
            return @ptrCast(extra.view().items(.@"0")[slice.start..][0..slice.len]);
        }
    };
};

pub const Key = union(enum) {
    int_type: IntType,
    ptr_type: PtrType,
    array_type: ArrayType,
    vector_type: VectorType,
    opt_type: Index,
    /// `anyframe->T`. The payload is the child type, which may be `none` to indicate
    /// `anyframe`.
    anyframe_type: Index,
    error_union_type: ErrorUnionType,
    simple_type: SimpleType,
    /// This represents a struct that has been explicitly declared in source code,
    /// or was created with `@Struct`. It is unique and based on a declaration.
    struct_type: ContainerType,
    /// This is a tuple type. Tuples are logically similar to structs, but have some
    /// important differences in semantics; they do not undergo staged type resolution,
    /// so cannot be self-referential, and they are not considered container/namespace
    /// types, so cannot have declarations and have structural equality properties.
    tuple_type: TupleType,
    union_type: ContainerType,
    opaque_type: ContainerType,
    enum_type: ContainerType,
    func_type: FuncType,
    error_set_type: ErrorSetType,
    /// The payload is the function body, either a `func_decl` or `func_instance`.
    inferred_error_set_type: Index,

    /// Typed `undefined`. This will never be `none`; untyped `undefined` is represented
    /// via `simple_value` and has a named `Index` tag for it.
    undef: Index,
    simple_value: SimpleValue,
    variable: Variable,
    @"extern": Extern,
    func: Func,
    int: Key.Int,
    err: Error,
    error_union: ErrorUnion,
    enum_literal: NullTerminatedString,
    /// A specific enum tag, indicated by the integer tag value.
    enum_tag: EnumTag,
    float: Float,
    ptr: Ptr,
    slice: Slice,
    opt: Opt,
    /// An instance of a struct, array, or vector.
    /// Each element/field stored as an `Index`.
    /// In the case of sentinel-terminated arrays, the sentinel value *is* stored,
    /// so the slice length will be one more than the type's array length.
    /// There must be at least one element which is not `undefined`. If all elements are
    /// undefined, instead create an undefined value of the aggregate type.
    aggregate: Aggregate,
    /// An instance of a union.
    un: Union,
    /// An instance of a `packed struct` or `packed union`.
    bitpack: Bitpack,

    /// A comptime function call with a memoized result.
    memoized_call: Key.MemoizedCall,

    pub const TypeValue = extern struct {
        ty: Index,
        val: Index,
    };

    pub const IntType = std.builtin.Type.Int;

    /// Extern for hashing via memory reinterpretation.
    pub const ErrorUnionType = extern struct {
        error_set_type: Index,
        payload_type: Index,
    };

    pub const ErrorSetType = struct {
        /// Set of error names, sorted by null terminated string index.
        names: NullTerminatedString.Slice,
        /// This is ignored by `get` but will always be provided by `indexToKey`.
        names_map: OptionalMapIndex = .none,

        /// Look up field index based on field name.
        pub fn nameIndex(self: ErrorSetType, ip: *const InternPool, name: NullTerminatedString) ?u32 {
            const map = self.names_map.unwrap().?.get(ip);
            const adapter: NullTerminatedString.Adapter = .{ .strings = self.names.get(ip) };
            const field_index = map.getIndexAdapted(name, adapter) orelse return null;
            return @intCast(field_index);
        }
    };

    /// Extern layout so it can be hashed with `std.mem.asBytes`.
    pub const PtrType = extern struct {
        child: Index,
        sentinel: Index = .none,
        flags: Flags = .{},
        packed_offset: PackedOffset = .{ .bit_offset = 0, .host_size = 0 },

        pub const VectorIndex = enum(u16) {
            none = std.math.maxInt(u16),
            _,
        };

        pub const Flags = packed struct(u32) {
            size: Size = .one,
            /// `none` indicates the ABI alignment of the pointee_type. In this
            /// case, this field *must* be set to `none`, otherwise the
            /// `InternPool` equality and hashing functions will return incorrect
            /// results.
            alignment: Alignment = .none,
            is_const: bool = false,
            is_volatile: bool = false,
            is_allowzero: bool = false,
            /// See src/target.zig defaultAddressSpace function for how to obtain
            /// an appropriate value for this field.
            address_space: AddressSpace = .generic,
            vector_index: VectorIndex = .none,
        };

        pub const PackedOffset = packed struct(u32) {
            /// If this is non-zero it means the pointer points to a sub-byte
            /// range of data, which is backed by a "host integer" with this
            /// number of bytes.
            /// When host_size=pointee_abi_size and bit_offset=0, this must be
            /// represented with host_size=0 instead.
            host_size: u16,
            bit_offset: u16,
        };

        pub const Size = std.builtin.Type.Pointer.Size;
        pub const AddressSpace = std.builtin.AddressSpace;
    };

    /// Extern so that hashing can be done via memory reinterpreting.
    pub const ArrayType = extern struct {
        len: u64,
        child: Index,
        sentinel: Index = .none,

        pub fn lenIncludingSentinel(array_type: ArrayType) u64 {
            return array_type.len + @intFromBool(array_type.sentinel != .none);
        }
    };

    /// Extern so that hashing can be done via memory reinterpreting.
    pub const VectorType = extern struct {
        len: u32,
        child: Index,
    };

    pub const TupleType = struct {
        types: Index.Slice,
        /// These elements may be `none`, indicating runtime-known.
        values: Index.Slice,
    };

    /// This is the hashmap key. To fetch other data associated with the type, see:
    /// * `loadStructType`
    /// * `loadUnionType`
    /// * `loadEnumType`
    /// * `loadOpaqueType`
    pub const ContainerType = union(enum) {
        /// This type corresponds to an actual source declaration, e.g. `struct { ... }`.
        /// It is hashed based on its ZIR instruction index and set of captures.
        declared: Declared,
        /// This type originates from a reification via `@Enum`, `@Struct`, `@Union` or from an anonymous initialization.
        /// It is hashed based on its ZIR instruction index and fields, attributes, etc.
        /// To avoid making this key overly complex, the type-specific data is hashed by Sema.
        reified: struct {
            /// A `reify`, `struct_init`, `struct_init_ref`, or `struct_init_anon` instruction.
            /// Alternatively, this is `main_struct_inst` of a ZON file.
            zir_index: TrackedInst.Index,
            /// A hash of this type's attributes, fields, etc, generated by Sema.
            type_hash: u64,
        },
        /// This type is an automatically-generated enum tag type for this union type.
        /// It is hashed based on the index of the union type it corresponds to.
        generated_union_tag: Index,

        pub const Declared = struct {
            /// A `struct_decl`, `union_decl`, `enum_decl`, or `opaque_decl` instruction.
            zir_index: TrackedInst.Index,
            /// The captured values of this type. These values must be fully resolved per the language spec.
            captures: union(enum) {
                owned: CaptureValue.Slice,
                external: []const CaptureValue,
            },
        };
    };

    pub const FuncType = struct {
        param_types: Index.Slice,
        return_type: Index,
        /// Tells whether a parameter is comptime. See `paramIsComptime` helper
        /// method for accessing this.
        comptime_bits: u32,
        /// Tells whether a parameter is noalias. See `paramIsNoalias` helper
        /// method for accessing this.
        noalias_bits: u32,
        cc: std.builtin.CallingConvention,
        is_var_args: bool,
        is_noinline: bool,

        pub fn paramIsComptime(self: @This(), i: u5) bool {
            assert(i < self.param_types.len);
            return @as(u1, @truncate(self.comptime_bits >> i)) != 0;
        }

        pub fn paramIsNoalias(self: @This(), i: u5) bool {
            assert(i < self.param_types.len);
            return @as(u1, @truncate(self.noalias_bits >> i)) != 0;
        }

        pub fn eql(a: FuncType, b: FuncType, ip: *const InternPool) bool {
            return std.mem.eql(Index, a.param_types.get(ip), b.param_types.get(ip)) and
                a.return_type == b.return_type and
                a.comptime_bits == b.comptime_bits and
                a.noalias_bits == b.noalias_bits and
                a.is_var_args == b.is_var_args and
                a.is_noinline == b.is_noinline and
                std.meta.eql(a.cc, b.cc);
        }

        pub fn hash(self: FuncType, hasher: *Hash, ip: *const InternPool) void {
            for (self.param_types.get(ip)) |param_type| {
                std.hash.autoHash(hasher, param_type);
            }
            std.hash.autoHash(hasher, self.return_type);
            std.hash.autoHash(hasher, self.comptime_bits);
            std.hash.autoHash(hasher, self.noalias_bits);
            std.hash.autoHash(hasher, self.cc);
            std.hash.autoHash(hasher, self.is_var_args);
            std.hash.autoHash(hasher, self.is_noinline);
        }
    };

    /// A runtime variable defined in this `Zcu`.
    pub const Variable = struct {
        ty: Index,
        init: Index,
        owner_nav: Nav.Index,
        is_threadlocal: bool,
    };

    pub const Extern = struct {
        /// The name of the extern symbol.
        name: NullTerminatedString,
        /// The type of the extern symbol itself.
        /// This may be `.anyopaque_type`, in which case the value may not be loaded.
        ty: Index,
        /// Library name if specified.
        /// For example `extern "c" fn write(...) usize` would have 'c' as library name.
        /// Index into the string table bytes.
        lib_name: OptionalNullTerminatedString,
        linkage: std.builtin.GlobalLinkage,
        visibility: std.builtin.SymbolVisibility,
        is_threadlocal: bool,
        is_dll_import: bool,
        relocation: std.builtin.ExternOptions.Relocation,
        decoration: ?std.builtin.ExternOptions.Decoration,
        is_const: bool,
        alignment: Alignment,
        @"addrspace": std.builtin.AddressSpace,
        /// The ZIR instruction which created this extern; used only for source locations.
        /// This is a `declaration`.
        zir_index: TrackedInst.Index,
        /// The `Nav` corresponding to this extern symbol.
        /// This is ignored by hashing and equality.
        owner_nav: Nav.Index,
        source: Tag.Extern.Flags.Source,
    };

    pub const Func = struct {
        tid: Zcu.PerThread.Id,
        /// In the case of a generic function, this type will potentially have fewer parameters
        /// than the generic owner's type, because the comptime parameters will be deleted.
        ty: Index,
        /// If this is a function body that has been coerced to a different type, for example
        /// ```
        /// fn f2() !void {}
        /// const f: fn()anyerror!void = f2;
        /// ```
        /// then it contains the original type of the function body.
        uncoerced_ty: Index,
        /// Index into extra array of the `FuncAnalysis` corresponding to this function.
        /// Used for mutating that data.
        analysis_extra_index: u32,
        /// Index into extra array of the `zir_body_inst` corresponding to this function.
        /// Used for mutating that data.
        zir_body_inst_extra_index: u32,
        /// Index into extra array of the resolved inferred error set for this function.
        /// Used for mutating that data.
        /// 0 when the function does not have an inferred error set.
        resolved_error_set_extra_index: u32,
        /// When a generic function is instantiated, branch_quota is inherited from the
        /// active Sema context. Importantly, this value is also updated when an existing
        /// generic function instantiation is found and called.
        /// This field contains the index into the extra array of this value,
        /// so that it can be mutated.
        /// This will be 0 when the function is not a generic function instantiation.
        branch_quota_extra_index: u32,
        owner_nav: Nav.Index,
        /// The ZIR instruction that is a function instruction. Use this to find
        /// the body. We store this rather than the body directly so that when ZIR
        /// is regenerated on update(), we can map this to the new corresponding
        /// ZIR instruction.
        zir_body_inst: TrackedInst.Index,
        /// Relative to owner Decl.
        lbrace_line: u32,
        /// Relative to owner Decl.
        rbrace_line: u32,
        lbrace_column: u32,
        rbrace_column: u32,

        /// The `func_decl` which is the generic function from whence this instance was spawned.
        /// If this is `none` it means the function is not a generic instantiation.
        generic_owner: Index,
        /// If this is a generic function instantiation, this will be non-empty.
        /// Corresponds to the parameters of the `generic_owner` type, which
        /// may have more parameters than `ty`.
        /// Each element is the comptime-known value the generic function was instantiated with,
        /// or `none` if the element is runtime-known.
        /// TODO: as a follow-up optimization, don't store `none` values here since that data
        /// is redundant with `comptime_bits` stored elsewhere.
        comptime_args: Index.Slice,

        /// Returns a pointer that becomes invalid after any additions to the `InternPool`.
        fn analysisPtr(func: Func, ip: *const InternPool) *FuncAnalysis {
            const extra = ip.getLocalShared(func.tid).extra.acquire();
            return @ptrCast(&extra.view().items(.@"0")[func.analysis_extra_index]);
        }

        pub fn analysisUnordered(func: Func, ip: *const InternPool) FuncAnalysis {
            return @atomicLoad(FuncAnalysis, func.analysisPtr(ip), .unordered);
        }

        pub fn setBranchHint(func: Func, ip: *InternPool, io: Io, hint: std.builtin.BranchHint) void {
            const extra_mutex = &ip.getLocal(func.tid).mutate.extra.mutex;
            extra_mutex.lockUncancelable(io);
            defer extra_mutex.unlock(io);

            const analysis_ptr = func.analysisPtr(ip);
            var analysis = analysis_ptr.*;
            analysis.branch_hint = hint;
            @atomicStore(FuncAnalysis, analysis_ptr, analysis, .release);
        }

        /// Returns a pointer that becomes invalid after any additions to the `InternPool`.
        fn zirBodyInstPtr(func: Func, ip: *const InternPool) *TrackedInst.Index {
            const extra = ip.getLocalShared(func.tid).extra.acquire();
            return @ptrCast(&extra.view().items(.@"0")[func.zir_body_inst_extra_index]);
        }

        pub fn zirBodyInstUnordered(func: Func, ip: *const InternPool) TrackedInst.Index {
            return @atomicLoad(TrackedInst.Index, func.zirBodyInstPtr(ip), .unordered);
        }

        /// Returns a pointer that becomes invalid after any additions to the `InternPool`.
        fn branchQuotaPtr(func: Func, ip: *const InternPool) *u32 {
            const extra = ip.getLocalShared(func.tid).extra.acquire();
            return &extra.view().items(.@"0")[func.branch_quota_extra_index];
        }

        pub fn branchQuotaUnordered(func: Func, ip: *const InternPool) u32 {
            return @atomicLoad(u32, func.branchQuotaPtr(ip), .unordered);
        }

        pub fn maxBranchQuota(func: Func, ip: *InternPool, io: Io, new_branch_quota: u32) void {
            const extra_mutex = &ip.getLocal(func.tid).mutate.extra.mutex;
            extra_mutex.lockUncancelable(io);
            defer extra_mutex.unlock(io);

            const branch_quota_ptr = func.branchQuotaPtr(ip);
            @atomicStore(u32, branch_quota_ptr, @max(branch_quota_ptr.*, new_branch_quota), .release);
        }

        /// Returns a pointer that becomes invalid after any additions to the `InternPool`.
        fn resolvedErrorSetPtr(func: Func, ip: *const InternPool) *Index {
            const extra = ip.getLocalShared(func.tid).extra.acquire();
            assert(func.analysisUnordered(ip).inferred_error_set);
            return @ptrCast(&extra.view().items(.@"0")[func.resolved_error_set_extra_index]);
        }

        pub fn resolvedErrorSetUnordered(func: Func, ip: *const InternPool) Index {
            return @atomicLoad(Index, func.resolvedErrorSetPtr(ip), .unordered);
        }

        pub fn setResolvedErrorSet(func: Func, ip: *InternPool, io: Io, ies: Index) void {
            const extra_mutex = &ip.getLocal(func.tid).mutate.extra.mutex;
            extra_mutex.lockUncancelable(io);
            defer extra_mutex.unlock(io);

            @atomicStore(Index, func.resolvedErrorSetPtr(ip), ies, .release);
        }
    };

    pub const Int = struct {
        ty: Index,
        storage: Storage,

        pub const Storage = union(enum) {
            u64: u64,
            i64: i64,
            big_int: BigIntConst,

            /// Big enough to fit any non-BigInt value
            pub const BigIntSpace = struct {
                /// The +1 is headroom so that operations such as incrementing once
                /// or decrementing once are possible without using an allocator.
                limbs: [(@sizeOf(u64) / @sizeOf(std.math.big.Limb)) + 1]std.math.big.Limb,
            };

            pub fn toBigInt(storage: Storage, space: *BigIntSpace) BigIntConst {
                return switch (storage) {
                    .big_int => |x| x,
                    inline .u64, .i64 => |x| BigIntMutable.init(&space.limbs, x).toConst(),
                };
            }
        };
    };

    pub const Error = extern struct {
        ty: Index,
        name: NullTerminatedString,
    };

    pub const ErrorUnion = struct {
        ty: Index,
        val: Value,

        pub const Value = union(enum) {
            err_name: NullTerminatedString,
            payload: Index,
        };
    };

    pub const EnumTag = extern struct {
        /// The enum type.
        ty: Index,
        /// The integer tag value which has the integer tag type of the enum.
        int: Index,
    };

    pub const Float = struct {
        ty: Index,
        /// The storage used must match the size of the float type being represented.
        storage: Storage,

        pub const Storage = union(enum) {
            f16: f16,
            f32: f32,
            f64: f64,
            f80: f80,
            f128: f128,
        };
    };

    pub const Ptr = struct {
        /// This is the pointer type, not the element type.
        ty: Index,
        /// The base address which this pointer is offset from.
        base_addr: BaseAddr,
        /// The offset of this pointer from `base_addr` in bytes.
        byte_offset: u64,

        pub const BaseAddr = union(enum) {
            const Tag = @typeInfo(BaseAddr).@"union".tag_type.?;

            /// Points to the value of a single `Nav`, which may be constant or a `variable`.
            nav: Nav.Index,

            /// Points to the value of a single comptime alloc stored in `Sema`.
            comptime_alloc: ComptimeAllocIndex,

            /// Points to a single unnamed constant value.
            uav: Uav,

            /// Points to a comptime field of a struct. Index is the field's value.
            ///
            /// TODO: this exists because these fields are semantically mutable. We
            /// should probably change the language so that this isn't the case.
            comptime_field: Index,

            /// A pointer with a fixed integer address, usually from `@ptrFromInt`.
            ///
            /// The address is stored entirely by `byte_offset`, which will be positive
            /// and in-range of a `usize`. The base address is, for all intents and purposes, 0.
            int,

            /// A pointer to the payload of an error union. Index is the error union pointer.
            /// To ensure a canonical representation, the type of the base pointer must:
            /// * be a one-pointer
            /// * be `const`, `volatile` and `allowzero`
            /// * have alignment 1
            /// * have the same address space as this pointer
            /// * have a host size, bit offset, and vector index of 0
            /// See `Value.canonicalizeBasePtr` which enforces these properties.
            eu_payload: Index,

            /// A pointer to the payload of a non-pointer-like optional. Index is the
            /// optional pointer. To ensure a canonical representation, the base
            /// pointer is subject to the same restrictions as in `eu_payload`.
            opt_payload: Index,

            /// A pointer to a field of a slice, or of an auto-layout struct or union. Slice fields
            /// are referenced according to `Value.slice_ptr_index` and `Value.slice_len_index`.
            /// Base is the aggregate pointer, which is subject to the same restrictions as
            /// in `eu_payload`.
            field: BaseIndex,

            /// A pointer to an element of a comptime-only array. Base is the
            /// many-pointer we are indexing into. It is subject to the same restrictions
            /// as in `eu_payload`, except it must be a many-pointer rather than a one-pointer.
            ///
            /// The element type of the base pointer must NOT be an array. Additionally, the
            /// base pointer is guaranteed to not be an `arr_elem` into a pointer with the
            /// same child type. Thus, since there are no two comptime-only types which are
            /// IMC to one another, the only case where the base pointer may also be an
            /// `arr_elem` is when this pointer is semantically invalid (e.g. it reinterprets
            /// a `type` as a `comptime_int`). These restrictions are in place to ensure
            /// a canonical representation.
            ///
            /// This kind of base address differs from others in that it may refer to any
            /// sequence of values; for instance, an `arr_elem` at index 2 may refer to
            /// any number of elements starting from index 2.
            ///
            /// Index must not be 0. To refer to the element at index 0, simply reinterpret
            /// the aggregate pointer.
            arr_elem: BaseIndex,

            pub const BaseIndex = struct {
                base: Index,
                index: u64,
            };
            pub const Uav = extern struct {
                val: Index,
                /// Contains the canonical pointer type of the anonymous
                /// declaration. This may equal `ty` of the `Ptr` or it may be
                /// different. Importantly, when lowering the anonymous decl,
                /// the original pointer type alignment must be used.
                orig_ty: Index,
            };

            pub fn eql(a: BaseAddr, b: BaseAddr) bool {
                if (@as(Key.Ptr.BaseAddr.Tag, a) != @as(Key.Ptr.BaseAddr.Tag, b)) return false;

                return switch (a) {
                    .nav => |a_nav| a_nav == b.nav,
                    .comptime_alloc => |a_alloc| a_alloc == b.comptime_alloc,
                    .uav => |ad| ad.val == b.uav.val and
                        ad.orig_ty == b.uav.orig_ty,
                    .int => true,
                    .eu_payload => |a_eu_payload| a_eu_payload == b.eu_payload,
                    .opt_payload => |a_opt_payload| a_opt_payload == b.opt_payload,
                    .comptime_field => |a_comptime_field| a_comptime_field == b.comptime_field,
                    .arr_elem => |a_elem| std.meta.eql(a_elem, b.arr_elem),
                    .field => |a_field| std.meta.eql(a_field, b.field),
                };
            }
        };
    };

    pub const Slice = struct {
        /// This is the slice type, not the element type.
        ty: Index,
        /// The slice's `ptr` field. Must be a many-ptr with the same properties as `ty`.
        ptr: Index,
        /// The slice's `len` field. Must be a `usize`.
        len: Index,
    };

    /// `null` is represented by the `val` field being `none`.
    pub const Opt = extern struct {
        /// This is the optional type; not the payload type.
        ty: Index,
        /// This could be `none`, indicating the optional is `null`.
        val: Index,
    };

    pub const Union = extern struct {
        /// This is the union type; not the field type.
        ty: Index,
        /// Indicates the active field. This could be `none`, which indicates the tag is not known. `none` is only a valid value for extern and packed unions.
        /// In those cases, the type of `val` is:
        ///   extern: a u8 array of the same byte length as the union
        ///   packed: an unsigned integer with the same bit size as the union
        tag: Index,
        /// The value of the active field.
        val: Index,
    };

    pub const Aggregate = struct {
        ty: Index,
        storage: Storage,

        pub const Storage = union(enum) {
            bytes: String,
            elems: []const Index,
            repeated_elem: Index,

            pub fn values(self: *const Storage) []const Index {
                return switch (self.*) {
                    .bytes => &.{},
                    .elems => |elems| elems,
                    .repeated_elem => |*elem| @as(*const [1]Index, elem),
                };
            }
        };
    };

    /// As well as a key, this type doubles as the payload in `extra` for `Tag.bitpack`.
    pub const Bitpack = struct {
        /// The `packed struct` or `packed union` type.
        ty: Index,
        /// The contents of the bitpack, represented as the backing integer value. The type of this
        /// value is the same as the backing integer type of `ty`.
        backing_int_val: Index,
    };

    pub const MemoizedCall = struct {
        func: Index,
        arg_values: []const Index,
        result: Index,
        branch_count: u32,
    };

    pub fn hash32(key: Key, ip: *const InternPool) u32 {
        return @truncate(key.hash64(ip));
    }

    pub fn hash64(key: Key, ip: *const InternPool) u64 {
        const asBytes = std.mem.asBytes;
        const KeyTag = @typeInfo(Key).@"union".tag_type.?;
        const seed = @intFromEnum(@as(KeyTag, key));
        return switch (key) {
            // TODO: assert no padding in these types
            inline .ptr_type,
            .array_type,
            .vector_type,
            .opt_type,
            .anyframe_type,
            .error_union_type,
            .simple_type,
            .simple_value,
            .opt,
            .undef,
            .err,
            .enum_literal,
            .enum_tag,
            .inferred_error_set_type,
            .un,
            => |x| Hash.hash(seed, asBytes(&x)),

            .int_type => |x| Hash.hash(seed + @intFromEnum(x.signedness), asBytes(&x.bits)),

            .error_union => |x| switch (x.val) {
                .err_name => |y| Hash.hash(seed + 0, asBytes(&x.ty) ++ asBytes(&y)),
                .payload => |y| Hash.hash(seed + 1, asBytes(&x.ty) ++ asBytes(&y)),
            },

            .variable => |variable| Hash.hash(seed, asBytes(&variable.owner_nav)),

            .opaque_type,
            .enum_type,
            .union_type,
            .struct_type,
            => |namespace_type| {
                var hasher = Hash.init(seed);
                std.hash.autoHash(&hasher, std.meta.activeTag(namespace_type));
                switch (namespace_type) {
                    .declared => |declared| {
                        std.hash.autoHash(&hasher, declared.zir_index);
                        const captures = switch (declared.captures) {
                            .owned => |cvs| cvs.get(ip),
                            .external => |cvs| cvs,
                        };
                        for (captures) |cv| {
                            std.hash.autoHash(&hasher, cv);
                        }
                    },
                    .reified => |reified| {
                        std.hash.autoHash(&hasher, reified.zir_index);
                        std.hash.autoHash(&hasher, reified.type_hash);
                    },
                    .generated_union_tag => |union_type| {
                        std.hash.autoHash(&hasher, union_type);
                    },
                }
                return hasher.final();
            },

            .int => |int| {
                var hasher = Hash.init(seed);
                // Canonicalize all integers by converting them to BigIntConst.
                var buffer: Key.Int.Storage.BigIntSpace = undefined;
                const big_int = int.storage.toBigInt(&buffer);

                std.hash.autoHash(&hasher, int.ty);
                std.hash.autoHash(&hasher, big_int.positive);
                for (big_int.limbs) |limb| std.hash.autoHash(&hasher, limb);
                return hasher.final();
            },

            .float => |float| {
                var hasher = Hash.init(seed);
                std.hash.autoHash(&hasher, float.ty);
                switch (float.storage) {
                    inline else => |val| std.hash.autoHash(
                        &hasher,
                        @as(std.meta.Int(.unsigned, @bitSizeOf(@TypeOf(val))), @bitCast(val)),
                    ),
                }
                return hasher.final();
            },

            .slice => |slice| Hash.hash(seed, asBytes(&slice.ty) ++ asBytes(&slice.ptr) ++ asBytes(&slice.len)),

            .ptr => |ptr| {
                // Int-to-ptr pointers are hashed separately than decl-referencing pointers.
                // This is sound due to pointer provenance rules.
                const addr_tag: Key.Ptr.BaseAddr.Tag = ptr.base_addr;
                const seed2 = seed + @intFromEnum(addr_tag);
                const big_offset: i128 = ptr.byte_offset;
                const common = asBytes(&ptr.ty) ++ asBytes(&big_offset);
                return switch (ptr.base_addr) {
                    inline .nav,
                    .comptime_alloc,
                    .uav,
                    .int,
                    .eu_payload,
                    .opt_payload,
                    .comptime_field,
                    => |x| Hash.hash(seed2, common ++ asBytes(&x)),

                    .arr_elem, .field => |x| Hash.hash(
                        seed2,
                        common ++ asBytes(&x.base) ++ asBytes(&x.index),
                    ),
                };
            },

            .aggregate => |aggregate| {
                var hasher = Hash.init(seed);
                std.hash.autoHash(&hasher, aggregate.ty);
                const len = ip.aggregateTypeLen(aggregate.ty);
                const child = switch (ip.indexToKey(aggregate.ty)) {
                    .array_type => |array_type| array_type.child,
                    .vector_type => |vector_type| vector_type.child,
                    .tuple_type, .struct_type => .none,
                    else => unreachable,
                };

                if (child == .u8_type) {
                    switch (aggregate.storage) {
                        .bytes => |bytes| for (bytes.toSlice(len, ip)) |byte| {
                            std.hash.autoHash(&hasher, KeyTag.int);
                            std.hash.autoHash(&hasher, byte);
                        },
                        .elems => |elems| for (elems[0..@intCast(len)]) |elem| {
                            const elem_key = ip.indexToKey(elem);
                            std.hash.autoHash(&hasher, @as(KeyTag, elem_key));
                            switch (elem_key) {
                                .undef => {},
                                .int => |int| std.hash.autoHash(
                                    &hasher,
                                    @as(u8, @intCast(int.storage.u64)),
                                ),
                                else => unreachable,
                            }
                        },
                        .repeated_elem => |elem| {
                            const elem_key = ip.indexToKey(elem);
                            var remaining = len;
                            while (remaining > 0) : (remaining -= 1) {
                                std.hash.autoHash(&hasher, @as(KeyTag, elem_key));
                                switch (elem_key) {
                                    .undef => {},
                                    .int => |int| std.hash.autoHash(
                                        &hasher,
                                        @as(u8, @intCast(int.storage.u64)),
                                    ),
                                    else => unreachable,
                                }
                            }
                        },
                    }
                    return hasher.final();
                }

                switch (aggregate.storage) {
                    .bytes => unreachable,
                    .elems => |elems| for (elems[0..@intCast(len)]) |elem|
                        std.hash.autoHash(&hasher, elem),
                    .repeated_elem => |elem| {
                        var remaining = len;
                        while (remaining > 0) : (remaining -= 1) std.hash.autoHash(&hasher, elem);
                    },
                }
                return hasher.final();
            },

            .error_set_type => |x| Hash.hash(seed, std.mem.sliceAsBytes(x.names.get(ip))),

            .tuple_type => |tuple_type| {
                var hasher = Hash.init(seed);
                for (tuple_type.types.get(ip)) |elem| std.hash.autoHash(&hasher, elem);
                for (tuple_type.values.get(ip)) |elem| std.hash.autoHash(&hasher, elem);
                return hasher.final();
            },

            .func_type => |func_type| {
                var hasher = Hash.init(seed);
                func_type.hash(&hasher, ip);
                return hasher.final();
            },

            .memoized_call => |memoized_call| {
                var hasher = Hash.init(seed);
                std.hash.autoHash(&hasher, memoized_call.func);
                for (memoized_call.arg_values) |arg| std.hash.autoHash(&hasher, arg);
                return hasher.final();
            },

            .func => |func| {
                // In the case of a function with an inferred error set, we
                // must not include the inferred error set type in the hash,
                // otherwise we would get false negatives for interning generic
                // function instances which have inferred error sets.

                if (func.generic_owner == .none and func.resolved_error_set_extra_index == 0) {
                    const bytes = asBytes(&func.owner_nav) ++ asBytes(&func.ty) ++
                        [1]u8{@intFromBool(func.uncoerced_ty == func.ty)};
                    return Hash.hash(seed, bytes);
                }

                var hasher = Hash.init(seed);
                std.hash.autoHash(&hasher, func.generic_owner);
                std.hash.autoHash(&hasher, func.uncoerced_ty == func.ty);
                for (func.comptime_args.get(ip)) |arg| std.hash.autoHash(&hasher, arg);
                if (func.resolved_error_set_extra_index == 0) {
                    std.hash.autoHash(&hasher, func.ty);
                } else {
                    var ty_info = ip.indexToFuncType(func.ty).?;
                    ty_info.return_type = ip.errorUnionPayload(ty_info.return_type);
                    ty_info.hash(&hasher, ip);
                }
                return hasher.final();
            },

            .@"extern" => |e| Hash.hash(seed, asBytes(&e.name) ++
                asBytes(&e.ty) ++ asBytes(&e.lib_name) ++
                asBytes(&e.linkage) ++ asBytes(&e.visibility) ++
                asBytes(&e.is_threadlocal) ++ asBytes(&e.is_dll_import) ++
                asBytes(&e.relocation) ++
                asBytes(&e.is_const) ++ asBytes(&e.alignment) ++ asBytes(&e.@"addrspace") ++
                asBytes(&e.zir_index) ++ &[1]u8{@intFromEnum(e.source)}),

            .bitpack => |bitpack| Hash.hash(seed, asBytes(&bitpack.ty) ++ asBytes(&bitpack.backing_int_val)),
        };
    }

    pub fn eql(a: Key, b: Key, ip: *const InternPool) bool {
        const KeyTag = @typeInfo(Key).@"union".tag_type.?;
        const a_tag: KeyTag = a;
        const b_tag: KeyTag = b;
        if (a_tag != b_tag) return false;
        switch (a) {
            .int_type => |a_info| {
                const b_info = b.int_type;
                return std.meta.eql(a_info, b_info);
            },
            .ptr_type => |a_info| {
                const b_info = b.ptr_type;
                return std.meta.eql(a_info, b_info);
            },
            .array_type => |a_info| {
                const b_info = b.array_type;
                return std.meta.eql(a_info, b_info);
            },
            .vector_type => |a_info| {
                const b_info = b.vector_type;
                return std.meta.eql(a_info, b_info);
            },
            .opt_type => |a_info| {
                const b_info = b.opt_type;
                return a_info == b_info;
            },
            .anyframe_type => |a_info| {
                const b_info = b.anyframe_type;
                return a_info == b_info;
            },
            .error_union_type => |a_info| {
                const b_info = b.error_union_type;
                return std.meta.eql(a_info, b_info);
            },
            .simple_type => |a_info| {
                const b_info = b.simple_type;
                return a_info == b_info;
            },
            .simple_value => |a_info| {
                const b_info = b.simple_value;
                return a_info == b_info;
            },
            .undef => |a_info| {
                const b_info = b.undef;
                return a_info == b_info;
            },
            .opt => |a_info| {
                const b_info = b.opt;
                return std.meta.eql(a_info, b_info);
            },
            .un => |a_info| {
                const b_info = b.un;
                return std.meta.eql(a_info, b_info);
            },
            .err => |a_info| {
                const b_info = b.err;
                return std.meta.eql(a_info, b_info);
            },
            .error_union => |a_info| {
                const b_info = b.error_union;
                return std.meta.eql(a_info, b_info);
            },
            .enum_literal => |a_info| {
                const b_info = b.enum_literal;
                return a_info == b_info;
            },
            .enum_tag => |a_info| {
                const b_info = b.enum_tag;
                return std.meta.eql(a_info, b_info);
            },
            .bitpack => |a_info| {
                const b_info = b.bitpack;
                return a_info.ty == b_info.ty and a_info.backing_int_val == b_info.backing_int_val;
            },

            .variable => |a_info| {
                const b_info = b.variable;
                return a_info.ty == b_info.ty and
                    a_info.init == b_info.init and
                    a_info.owner_nav == b_info.owner_nav and
                    a_info.is_threadlocal == b_info.is_threadlocal;
            },
            .@"extern" => |a_info| {
                const b_info = b.@"extern";
                return a_info.name == b_info.name and
                    a_info.ty == b_info.ty and
                    a_info.lib_name == b_info.lib_name and
                    a_info.linkage == b_info.linkage and
                    a_info.visibility == b_info.visibility and
                    a_info.is_threadlocal == b_info.is_threadlocal and
                    a_info.is_dll_import == b_info.is_dll_import and
                    a_info.relocation == b_info.relocation and
                    a_info.is_const == b_info.is_const and
                    a_info.alignment == b_info.alignment and
                    a_info.@"addrspace" == b_info.@"addrspace" and
                    a_info.zir_index == b_info.zir_index and
                    a_info.source == b_info.source;
            },
            .func => |a_info| {
                const b_info = b.func;

                if (a_info.generic_owner != b_info.generic_owner)
                    return false;

                if (a_info.generic_owner == .none) {
                    if (a_info.owner_nav != b_info.owner_nav)
                        return false;
                } else {
                    if (!std.mem.eql(
                        Index,
                        a_info.comptime_args.get(ip),
                        b_info.comptime_args.get(ip),
                    )) return false;
                }

                if ((a_info.ty == a_info.uncoerced_ty) !=
                    (b_info.ty == b_info.uncoerced_ty))
                {
                    return false;
                }

                if (a_info.ty == b_info.ty)
                    return true;

                // There is one case where the types may be inequal but we
                // still want to find the same function body instance. In the
                // case of the functions having an inferred error set, the key
                // used to find an existing function body will necessarily have
                // a unique inferred error set type, because it refers to the
                // function body InternPool Index. To make this case work we
                // omit the inferred error set from the equality check.
                if (a_info.resolved_error_set_extra_index == 0 or
                    b_info.resolved_error_set_extra_index == 0)
                {
                    return false;
                }
                var a_ty_info = ip.indexToFuncType(a_info.ty).?;
                a_ty_info.return_type = ip.errorUnionPayload(a_ty_info.return_type);
                var b_ty_info = ip.indexToFuncType(b_info.ty).?;
                b_ty_info.return_type = ip.errorUnionPayload(b_ty_info.return_type);
                return a_ty_info.eql(b_ty_info, ip);
            },

            .slice => |a_info| {
                const b_info = b.slice;
                if (a_info.ty != b_info.ty) return false;
                if (a_info.ptr != b_info.ptr) return false;
                if (a_info.len != b_info.len) return false;
                return true;
            },

            .ptr => |a_info| {
                const b_info = b.ptr;
                if (a_info.ty != b_info.ty) return false;
                if (a_info.byte_offset != b_info.byte_offset) return false;
                if (!a_info.base_addr.eql(b_info.base_addr)) return false;
                return true;
            },

            .int => |a_info| {
                const b_info = b.int;

                if (a_info.ty != b_info.ty)
                    return false;

                return switch (a_info.storage) {
                    .u64 => |aa| switch (b_info.storage) {
                        .u64 => |bb| aa == bb,
                        .i64 => |bb| aa == bb,
                        .big_int => |bb| bb.orderAgainstScalar(aa) == .eq,
                    },
                    .i64 => |aa| switch (b_info.storage) {
                        .u64 => |bb| aa == bb,
                        .i64 => |bb| aa == bb,
                        .big_int => |bb| bb.orderAgainstScalar(aa) == .eq,
                    },
                    .big_int => |aa| switch (b_info.storage) {
                        .u64 => |bb| aa.orderAgainstScalar(bb) == .eq,
                        .i64 => |bb| aa.orderAgainstScalar(bb) == .eq,
                        .big_int => |bb| aa.eql(bb),
                    },
                };
            },

            .float => |a_info| {
                const b_info = b.float;

                if (a_info.ty != b_info.ty)
                    return false;

                if (a_info.ty == .c_longdouble_type and a_info.storage != .f80) {
                    // These are strange: we'll sometimes represent them as f128, even if the
                    // underlying type is smaller. f80 is an exception: see float_c_longdouble_f80.
                    const a_val: u128 = switch (a_info.storage) {
                        inline else => |val| @bitCast(@as(f128, @floatCast(val))),
                    };
                    const b_val: u128 = switch (b_info.storage) {
                        inline else => |val| @bitCast(@as(f128, @floatCast(val))),
                    };
                    return a_val == b_val;
                }

                const StorageTag = @typeInfo(Key.Float.Storage).@"union".tag_type.?;
                assert(@as(StorageTag, a_info.storage) == @as(StorageTag, b_info.storage));

                switch (a_info.storage) {
                    inline else => |val, tag| {
                        const Bits = std.meta.Int(.unsigned, @bitSizeOf(@TypeOf(val)));
                        const a_bits: Bits = @bitCast(val);
                        const b_bits: Bits = @bitCast(@field(b_info.storage, @tagName(tag)));
                        return a_bits == b_bits;
                    },
                }
            },

            inline .opaque_type, .enum_type, .union_type, .struct_type => |a_info, a_tag_ct| {
                const b_info = @field(b, @tagName(a_tag_ct));
                if (std.meta.activeTag(a_info) != b_info) return false;
                switch (a_info) {
                    .declared => |a_d| {
                        const b_d = b_info.declared;
                        if (a_d.zir_index != b_d.zir_index) return false;
                        const a_captures = switch (a_d.captures) {
                            .owned => |s| s.get(ip),
                            .external => |cvs| cvs,
                        };
                        const b_captures = switch (b_d.captures) {
                            .owned => |s| s.get(ip),
                            .external => |cvs| cvs,
                        };
                        return std.mem.eql(u32, @ptrCast(a_captures), @ptrCast(b_captures));
                    },
                    .reified => |a_r| {
                        const b_r = b_info.reified;
                        return a_r.zir_index == b_r.zir_index and
                            a_r.type_hash == b_r.type_hash;
                    },
                    .generated_union_tag => |a_union_ty| return a_union_ty == b_info.generated_union_tag,
                }
            },
            .aggregate => |a_info| {
                const b_info = b.aggregate;
                if (a_info.ty != b_info.ty) return false;

                const len = ip.aggregateTypeLen(a_info.ty);
                const StorageTag = @typeInfo(Key.Aggregate.Storage).@"union".tag_type.?;
                if (@as(StorageTag, a_info.storage) != @as(StorageTag, b_info.storage)) {
                    for (0..@intCast(len)) |elem_index| {
                        const a_elem = switch (a_info.storage) {
                            .bytes => |bytes| ip.getIfExists(.{ .int = .{
                                .ty = .u8_type,
                                .storage = .{ .u64 = bytes.at(elem_index, ip) },
                            } }) orelse return false,
                            .elems => |elems| elems[elem_index],
                            .repeated_elem => |elem| elem,
                        };
                        const b_elem = switch (b_info.storage) {
                            .bytes => |bytes| ip.getIfExists(.{ .int = .{
                                .ty = .u8_type,
                                .storage = .{ .u64 = bytes.at(elem_index, ip) },
                            } }) orelse return false,
                            .elems => |elems| elems[elem_index],
                            .repeated_elem => |elem| elem,
                        };
                        if (a_elem != b_elem) return false;
                    }
                    return true;
                }

                switch (a_info.storage) {
                    .bytes => |a_bytes| {
                        const b_bytes = b_info.storage.bytes;
                        return a_bytes == b_bytes or
                            std.mem.eql(u8, a_bytes.toSlice(len, ip), b_bytes.toSlice(len, ip));
                    },
                    .elems => |a_elems| {
                        const b_elems = b_info.storage.elems;
                        return std.mem.eql(
                            Index,
                            a_elems[0..@intCast(len)],
                            b_elems[0..@intCast(len)],
                        );
                    },
                    .repeated_elem => |a_elem| {
                        const b_elem = b_info.storage.repeated_elem;
                        return a_elem == b_elem;
                    },
                }
            },
            .tuple_type => |a_info| {
                const b_info = b.tuple_type;
                return std.mem.eql(Index, a_info.types.get(ip), b_info.types.get(ip)) and
                    std.mem.eql(Index, a_info.values.get(ip), b_info.values.get(ip));
            },
            .error_set_type => |a_info| {
                const b_info = b.error_set_type;
                return std.mem.eql(NullTerminatedString, a_info.names.get(ip), b_info.names.get(ip));
            },
            .inferred_error_set_type => |a_info| {
                const b_info = b.inferred_error_set_type;
                return a_info == b_info;
            },

            .func_type => |a_info| {
                const b_info = b.func_type;
                return Key.FuncType.eql(a_info, b_info, ip);
            },

            .memoized_call => |a_info| {
                const b_info = b.memoized_call;
                return a_info.func == b_info.func and
                    std.mem.eql(Index, a_info.arg_values, b_info.arg_values);
            },
        }
    }

    pub fn typeOf(key: Key) Index {
        return switch (key) {
            .int_type,
            .ptr_type,
            .array_type,
            .vector_type,
            .opt_type,
            .anyframe_type,
            .error_union_type,
            .error_set_type,
            .inferred_error_set_type,
            .simple_type,
            .struct_type,
            .union_type,
            .opaque_type,
            .enum_type,
            .tuple_type,
            .func_type,
            => .type_type,

            inline .ptr,
            .slice,
            .int,
            .float,
            .opt,
            .variable,
            .@"extern",
            .func,
            .err,
            .error_union,
            .enum_tag,
            .aggregate,
            .un,
            .bitpack,
            => |x| x.ty,

            .enum_literal => .enum_literal_type,

            .undef => |x| x,

            .simple_value => |s| switch (s) {
                .void => .void_type,
                .null => .null_type,
                .false, .true => .bool_type,
                .@"unreachable" => .noreturn_type,
            },

            .memoized_call => unreachable,
        };
    }
};

pub const LoadedStructType = struct {
    /// Index of the `struct_decl` or `reify` ZIR instruction.
    zir_index: TrackedInst.Index,
    captures: CaptureValue.Slice,
    is_reified: bool,

    // TODO: the non-fqn will be needed by the new dwarf structure
    /// The name of this struct type.
    name: NullTerminatedString,
    /// If this is a declared type with the `.parent` name strategy, this is the `Nav` it was named after.
    /// Otherwise, or if this is a file's root struct type, this is `.none`.
    name_nav: Nav.Index.Optional,
    namespace: NamespaceIndex,

    layout: std.builtin.Type.ContainerLayout,
    /// May be `undefined` if `layout != .@"packed"`.
    packed_backing_mode: BackingTypeMode,

    /// Initially `false`, and set to `true` once any dependency on or reference to the struct's
    /// layout is encountered, after which it is never reset to `false`, even across incremental
    /// updates.
    ///
    /// This field is purely an optimization to avoid resolving the layout of types whose layouts
    /// are never demanded. If this field is `true` but the layout is not actually needed, the
    /// compiler frontend resolves this by traversing the reference graph at the end of each update
    /// with `Zcu.resolveReferences` and hiding compile errors which arise from this analysis.
    want_layout: bool,

    // The remaining fields are only valid once the struct's layout is resolved.
    field_name_map: MapIndex,
    field_names: NullTerminatedString.Slice,
    field_types: Index.Slice,
    field_defaults: Index.Slice,
    field_aligns: Alignment.Slice,
    field_is_comptime_bits: ComptimeBits,
    field_runtime_order: RuntimeOrder.Slice,
    field_offsets: Offsets,
    packed_backing_int_type: Index,
    class: TypeClass,
    size: u32,
    alignment: Alignment,

    pub const ComptimeBits = struct {
        tid: Zcu.PerThread.Id,
        start: u32,
        /// This is the number of u32 elements, not the number of struct fields.
        len: u32,

        pub const empty: ComptimeBits = .{ .tid = .main, .start = 0, .len = 0 };

        pub fn getAll(this: ComptimeBits, ip: *const InternPool) []u32 {
            const extra = ip.getLocalShared(this.tid).extra.acquire();
            return extra.view().items(.@"0")[this.start..][0..this.len];
        }

        pub fn get(this: ComptimeBits, ip: *const InternPool, i: usize) bool {
            if (this.len == 0) return false;
            return @as(u1, @truncate(this.getAll(ip)[i / 32] >> @intCast(i % 32))) != 0;
        }
    };

    pub const Offsets = struct {
        tid: Zcu.PerThread.Id,
        start: u32,
        len: u32,

        pub const empty: Offsets = .{ .tid = .main, .start = 0, .len = 0 };

        pub fn get(this: Offsets, ip: *const InternPool) []u32 {
            const extra = ip.getLocalShared(this.tid).extra.acquire();
            return @ptrCast(extra.view().items(.@"0")[this.start..][0..this.len]);
        }
    };

    pub const RuntimeOrder = enum(u32) {
        /// Placeholder until layout is resolved.
        unresolved = std.math.maxInt(u32) - 0,
        /// Field not present at runtime
        omitted = std.math.maxInt(u32) - 1,
        _,

        pub const Slice = struct {
            tid: Zcu.PerThread.Id,
            start: u32,
            len: u32,

            pub const empty: Slice = .{ .tid = .main, .start = 0, .len = 0 };

            pub fn get(slice: RuntimeOrder.Slice, ip: *const InternPool) []RuntimeOrder {
                const extra = ip.getLocalShared(slice.tid).extra.acquire();
                return @ptrCast(extra.view().items(.@"0")[slice.start..][0..slice.len]);
            }
        };

        pub fn toInt(i: RuntimeOrder) ?u32 {
            return switch (i) {
                .omitted => null,
                .unresolved => unreachable,
                else => @intFromEnum(i),
            };
        }
    };

    /// Look up field index based on field name.
    pub fn nameIndex(s: LoadedStructType, ip: *const InternPool, name: NullTerminatedString) ?u32 {
        const map = s.field_name_map.get(ip);
        const adapter: NullTerminatedString.Adapter = .{ .strings = s.field_names.get(ip) };
        const field_index = map.getIndexAdapted(name, adapter) orelse return null;
        return @intCast(field_index);
    }

    /// Iterates over non-comptime fields in the order they are laid out in memory at runtime.
    /// May or may not include zero-bit fields.
    /// Asserts the struct is not packed.
    pub fn iterateRuntimeOrder(s: *const LoadedStructType, ip: *InternPool) RuntimeOrderIterator {
        switch (s.layout) {
            .auto => {
                const ro = std.mem.sliceTo(s.field_runtime_order.get(ip), .omitted);
                return .{
                    .runtime_order = ro,
                    .fields_len = @intCast(ro.len),
                    .next_index = 0,
                };
            },
            .@"extern" => return .{
                .runtime_order = null,
                .fields_len = s.field_names.len,
                .next_index = 0,
            },
            .@"packed" => unreachable,
        }
    }
    pub const RuntimeOrderIterator = struct {
        runtime_order: ?[]const RuntimeOrder,
        fields_len: u32,
        next_index: u32,
        pub fn next(it: *RuntimeOrderIterator) ?u32 {
            const i = it.next_index;
            if (i == it.fields_len) return null;
            it.next_index = i + 1;
            const ro = it.runtime_order orelse return i;
            return ro[i].toInt().?;
        }
    };

    pub fn iterateRuntimeOrderReverse(s: *const LoadedStructType, ip: *InternPool) ReverseRuntimeOrderIterator {
        switch (s.layout) {
            .auto => {
                const ro = std.mem.sliceTo(s.field_runtime_order.get(ip), .omitted);
                return .{
                    .runtime_order = ro,
                    .last_index = @intCast(ro.len),
                };
            },
            .@"extern" => return .{
                .runtime_order = null,
                .last_index = s.field_names.len,
            },
            .@"packed" => unreachable,
        }
    }
    pub const ReverseRuntimeOrderIterator = struct {
        runtime_order: ?[]const RuntimeOrder,
        last_index: u32,
        pub fn next(it: *ReverseRuntimeOrderIterator) ?u32 {
            if (it.last_index == 0) return null;
            const i = it.last_index - 1;
            it.last_index = i;
            const ro = it.runtime_order orelse return i;
            return ro[i].toInt().?;
        }
    };
};

/// Unlike `Tag.TypeUnion` which is an encoding, and `Key.UnionType` which is a
/// minimal hashmap key, this type is a convenience type that contains info
/// needed by semantic analysis.
pub const LoadedUnionType = struct {
    /// Index of the `union_decl` or `reify` ZIR instruction.
    zir_index: TrackedInst.Index,
    captures: CaptureValue.Slice,
    is_reified: bool,

    // TODO: the non-fqn will be needed by the new dwarf structure
    /// The name of this union type.
    name: NullTerminatedString,
    /// If this is a declared type with the `.parent` name strategy, this is the `Nav` it was named after.
    /// Otherwise, this is `.none`.
    name_nav: Nav.Index.Optional,
    namespace: NamespaceIndex,

    layout: std.builtin.Type.ContainerLayout,
    enum_tag_mode: BackingTypeMode,
    /// May be `undefined` if `layout != .@"packed"`.
    packed_backing_mode: BackingTypeMode,

    /// Only reified unions store field names; typically they should be loaded from `enum_tag_type`
    /// instead. Reified unions store them because type resolution needs them in order to validate
    /// or populate `enum_tag_type`.
    reified_field_names: NullTerminatedString.Slice,

    /// Initially `false`, and set to `true` once any dependency on or reference to the struct's
    /// layout is encountered, after which it is never reset to `false`, even across incremental
    /// updates.
    ///
    /// This field is purely an optimization to avoid resolving the layout of types whose layouts
    /// are never demanded. If this field is `true` but the layout is not actually needed, the
    /// compiler frontend resolves this by traversing the reference graph at the end of each update
    /// with `Zcu.resolveReferences` and hiding compile errors which arise from this analysis.
    want_layout: bool,

    // The remaining fields are only valid once the union's layout is resolved.
    field_types: Index.Slice,
    field_aligns: Alignment.Slice,
    tag_usage: TagUsage,
    /// While `tag_usage` indicates whether the union should logically contain a tag, it may be
    /// omitted if the union layout is resolved as OPV or NPV. This field is `true` iff there is an
    /// actual runtime tag in the union layout.
    has_runtime_tag: bool,
    /// Even if `tag_usage == .none` and `has_runtime_tag == false`, this is still populated with
    /// the union's "hypothetical" tag type.
    enum_tag_type: Index,
    packed_backing_int_type: Index,
    class: TypeClass,
    size: u32,
    padding: u32,
    alignment: Alignment,

    pub const TagUsage = enum(u2) {
        none,
        safety,
        tagged,
    };
};

pub const LoadedEnumType = struct {
    /// This is `none` iff this is a generated tag type.
    /// Otherwise, index of the `enum_decl` or `reify` ZIR instruction.
    zir_index: TrackedInst.Index.Optional,
    captures: CaptureValue.Slice,
    /// If `zir_index` is `.none`, this is the union type for which this enum is the tag type.
    owner_union: Index,
    is_reified: bool,

    // TODO: the non-fqn will be needed by the new dwarf structure
    /// The name of this enum type.
    name: NullTerminatedString,
    /// If this is a declared type with the `.parent` name strategy, this is the `Nav` it was named after.
    /// Otherwise, this is `.none`.
    name_nav: Nav.Index.Optional,
    namespace: NamespaceIndex,

    int_tag_mode: BackingTypeMode,
    nonexhaustive: bool,

    /// Initially `false`, and set to `true` once any dependency on or reference to the struct's
    /// layout is encountered, after which it is never reset to `false`, even across incremental
    /// updates.
    ///
    /// This field is purely an optimization to avoid resolving the layout of types whose layouts
    /// are never demanded. If this field is `true` but the layout is not actually needed, the
    /// compiler frontend resolves this by traversing the reference graph at the end of each update
    /// with `Zcu.resolveReferences` and hiding compile errors which arise from this analysis.
    want_layout: bool,

    // The remaining fields are only valid once the enum's layout is resolved.
    int_tag_type: Index,
    field_name_map: MapIndex,
    field_names: NullTerminatedString.Slice,
    field_value_map: OptionalMapIndex,
    field_values: Index.Slice,

    /// Look up field index based on field name.
    pub fn nameIndex(e: LoadedEnumType, ip: *const InternPool, name: NullTerminatedString) ?u32 {
        const map = e.field_name_map.get(ip);
        const adapter: NullTerminatedString.Adapter = .{ .strings = e.field_names.get(ip) };
        const field_index = map.getIndexAdapted(name, adapter) orelse return null;
        return @intCast(field_index);
    }

    /// Look up field index based on integer tag value.
    /// Asserts that the type of `tag_val` is `enum_obj.int_tag_type`.
    /// Asserts that `tag_val` is not `undefined`.
    pub fn tagValueIndex(e: LoadedEnumType, ip: *const InternPool, tag_val: Index) ?u32 {
        assert(ip.typeOf(tag_val) == e.int_tag_type);
        assert(ip.indexToKey(tag_val) == .int);
        if (e.field_value_map.unwrap()) |field_value_map| {
            const map = field_value_map.get(ip);
            const adapter: Index.Adapter = .{ .indexes = e.field_values.get(ip) };
            const field_index = map.getIndexAdapted(tag_val, adapter) orelse return null;
            return @intCast(field_index);
        }
        // Auto-numbered enum, so convert `tag_val` to field index
        const field_index = switch (ip.indexToKey(tag_val).int.storage) {
            inline .u64, .i64 => |x| std.math.cast(u32, x) orelse return null,
            .big_int => |x| x.toInt(u32) catch return null,
        };
        return if (field_index < e.field_names.len) field_index else null;
    }
};

pub const LoadedOpaqueType = struct {
    /// Index of the `opaque_decl` instruction.
    zir_index: TrackedInst.Index,
    captures: CaptureValue.Slice,

    // TODO: the non-fqn will be needed by the new dwarf structure
    /// The name of this opaque type.
    name: NullTerminatedString,
    /// If this is a declared type with the `.parent` name strategy, this is the `Nav` it was named after.
    /// Otherwise, this is `.none`.
    name_nav: Nav.Index.Optional,
    namespace: NamespaceIndex,
};

pub fn loadStructType(ip: *const InternPool, index: Index) LoadedStructType {
    const unwrapped_index = index.unwrap(ip);
    const extra_list = unwrapped_index.getExtra(ip);
    const extra_items = extra_list.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    // Exiting this `switch` means this is a `packed struct`.
    const backing_mode: BackingTypeMode, const any_defaults: bool = switch (item.tag) {
        .type_struct_packed_auto => .{ .auto, false },
        .type_struct_packed_explicit => .{ .explicit, false },
        .type_struct_packed_auto_defaults => .{ .auto, true },
        .type_struct_packed_explicit_defaults => .{ .explicit, true },
        .type_struct => {
            const extra = extraDataTrail(extra_list, Tag.TypeStruct, item.data);
            var extra_index = extra.end;
            const captures: CaptureValue.Slice = switch (extra.data.flags.any_captures) {
                .reified => captures: {
                    extra_index += 2; // type_hash: PackedU64
                    break :captures .empty;
                },
                .false => .empty,
                .true => captures: {
                    const len = extra_items[extra_index];
                    extra_index += 1;
                    break :captures .{
                        .tid = unwrapped_index.tid,
                        .start = extra_index,
                        .len = len,
                    };
                },
            };
            extra_index += captures.len;
            const field_names: NullTerminatedString.Slice = .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            };
            extra_index += field_names.len;
            const field_types: Index.Slice = .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            };
            extra_index += field_types.len;
            const field_defaults: Index.Slice = if (extra.data.flags.any_field_defaults) .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            } else .empty;
            extra_index += field_defaults.len;
            const field_aligns: Alignment.Slice = if (extra.data.flags.any_field_aligns) .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            } else .empty;
            extra_index += std.math.divCeil(u32, field_aligns.len, 4) catch unreachable;
            const field_is_comptime_bits: LoadedStructType.ComptimeBits = if (extra.data.flags.any_comptime_fields) .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = std.math.divCeil(u32, extra.data.fields_len, 32) catch unreachable,
            } else .empty;
            extra_index += field_is_comptime_bits.len;
            const field_runtime_order: LoadedStructType.RuntimeOrder.Slice = if (extra.data.flags.layout == .auto) .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            } else .empty;
            extra_index += field_runtime_order.len;
            const field_offsets: LoadedStructType.Offsets = .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            };
            extra_index += field_offsets.len;

            return .{
                .zir_index = extra.data.zir_index,
                .captures = captures,
                .is_reified = extra.data.flags.any_captures == .reified,
                .name = extra.data.name,
                .name_nav = extra.data.name_nav,
                .namespace = extra.data.namespace,
                .layout = switch (extra.data.flags.layout) {
                    .auto => .auto,
                    .@"extern" => .@"extern",
                },
                .packed_backing_mode = undefined,

                .want_layout = extra.data.flags.want_layout,

                .field_name_map = extra.data.field_name_map,
                .field_names = field_names,
                .field_types = field_types,
                .field_defaults = field_defaults,
                .field_aligns = field_aligns,
                .field_is_comptime_bits = field_is_comptime_bits,
                .field_runtime_order = field_runtime_order,
                .field_offsets = field_offsets,
                .packed_backing_int_type = .none,
                .class = extra.data.flags.class,
                .size = extra.data.size,
                .alignment = extra.data.flags.alignment,
            };
        },
        else => unreachable,
    };
    const extra = extraDataTrail(extra_list, Tag.TypeStructPacked, item.data);
    var extra_index = extra.end;
    const captures: CaptureValue.Slice = switch (extra.data.bits.captures_len) {
        .reified => captures: {
            extra_index += 2; // type_hash: PackedU64
            break :captures .empty;
        },
        _ => |n| .{
            .tid = unwrapped_index.tid,
            .start = extra_index,
            .len = @intFromEnum(n),
        },
    };
    extra_index += captures.len;
    const field_names: NullTerminatedString.Slice = .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    };
    extra_index += field_names.len;
    const field_types: Index.Slice = .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    };
    extra_index += field_types.len;
    const field_defaults: Index.Slice = if (any_defaults) .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    } else .empty;
    extra_index += field_defaults.len;
    return .{
        .zir_index = extra.data.zir_index,
        .captures = captures,
        .is_reified = extra.data.bits.captures_len == .reified,
        .name = extra.data.name,
        .name_nav = extra.data.name_nav,
        .namespace = extra.data.namespace,
        .layout = .@"packed",
        .packed_backing_mode = backing_mode,

        .want_layout = extra.data.bits.want_layout,

        .field_name_map = extra.data.field_name_map,
        .field_names = field_names,
        .field_types = field_types,
        .field_defaults = field_defaults,
        .field_aligns = .empty,
        .field_is_comptime_bits = .empty,
        .field_runtime_order = .empty,
        .field_offsets = .empty,
        .packed_backing_int_type = extra.data.backing_int_type,
        .class = undefined,
        .size = undefined,
        .alignment = undefined,
    };
}

pub fn loadUnionType(ip: *const InternPool, index: Index) LoadedUnionType {
    const unwrapped_index = index.unwrap(ip);
    const extra_list = unwrapped_index.getExtra(ip);
    const extra_items = extra_list.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    // Exiting this `switch` means this is a `packed union`.
    const backing_mode: BackingTypeMode = switch (item.tag) {
        .type_union_packed_auto => .auto,
        .type_union_packed_explicit => .explicit,
        .type_union => {
            const extra = extraDataTrail(extra_list, Tag.TypeUnion, item.data);
            var extra_index = extra.end;
            const captures: CaptureValue.Slice = switch (extra.data.flags.any_captures) {
                .reified => captures: {
                    extra_index += 2; // type_hash: PackedU64
                    break :captures .empty;
                },
                .false => .empty,
                .true => captures: {
                    const len = extra_items[extra_index];
                    extra_index += 1;
                    break :captures .{
                        .tid = unwrapped_index.tid,
                        .start = extra_index,
                        .len = len,
                    };
                },
            };
            extra_index += captures.len;
            const reified_field_names: NullTerminatedString.Slice = if (extra.data.flags.any_captures == .reified) .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            } else .empty;
            extra_index += reified_field_names.len;
            const field_types: Index.Slice = .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            };
            extra_index += field_types.len;
            const field_aligns: Alignment.Slice = if (extra.data.flags.any_field_aligns) .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = extra.data.fields_len,
            } else .empty;
            extra_index += std.math.divCeil(u32, field_aligns.len, 4) catch unreachable;

            return .{
                .zir_index = extra.data.zir_index,
                .captures = captures,
                .is_reified = extra.data.flags.any_captures == .reified,
                .name = extra.data.name,
                .name_nav = extra.data.name_nav,
                .namespace = extra.data.namespace,
                .layout = switch (extra.data.flags.layout) {
                    .auto => .auto,
                    .@"extern" => .@"extern",
                },
                .tag_usage = extra.data.flags.tag_usage,
                .enum_tag_mode = extra.data.flags.enum_tag_mode,
                .enum_tag_type = extra.data.enum_tag_type,
                .packed_backing_mode = undefined,
                .packed_backing_int_type = undefined,
                .reified_field_names = reified_field_names,
                .want_layout = extra.data.flags.want_layout,
                .field_types = field_types,
                .field_aligns = field_aligns,
                .has_runtime_tag = extra.data.flags.has_runtime_tag,
                .class = extra.data.flags.class,
                .size = extra.data.size,
                .padding = extra.data.padding,
                .alignment = extra.data.flags.alignment,
            };
        },
        else => unreachable,
    };
    const extra = extraDataTrail(extra_list, Tag.TypeUnionPacked, item.data);
    var extra_index = extra.end;
    const captures: CaptureValue.Slice = switch (extra.data.bits.captures_len) {
        .reified => captures: {
            extra_index += 2; // type_hash: PackedU64
            break :captures .empty;
        },
        _ => |n| .{
            .tid = unwrapped_index.tid,
            .start = extra_index,
            .len = @intFromEnum(n),
        },
    };
    extra_index += captures.len;
    const reified_field_names: NullTerminatedString.Slice = if (extra.data.bits.captures_len == .reified) .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    } else .empty;
    extra_index += reified_field_names.len;
    const field_types: Index.Slice = .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    };
    extra_index += field_types.len;
    return .{
        .zir_index = extra.data.zir_index,
        .captures = captures,
        .is_reified = extra.data.bits.captures_len == .reified,
        .name = extra.data.name,
        .name_nav = extra.data.name_nav,
        .namespace = extra.data.namespace,
        .layout = .@"packed",
        .tag_usage = .none,
        .enum_tag_mode = .auto,
        .enum_tag_type = extra.data.enum_tag_type,
        .packed_backing_mode = backing_mode,
        .packed_backing_int_type = extra.data.backing_int_type,
        .reified_field_names = reified_field_names,
        .want_layout = extra.data.bits.want_layout,
        .field_types = field_types,
        .field_aligns = .empty,
        .has_runtime_tag = undefined,
        .class = undefined,
        .size = undefined,
        .padding = undefined,
        .alignment = undefined,
    };
}

pub fn loadEnumType(ip: *const InternPool, index: Index) LoadedEnumType {
    const unwrapped_index = index.unwrap(ip);
    const extra_list = unwrapped_index.getExtra(ip);
    const extra_items = extra_list.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    const explicit_int_tag: bool, const nonexhaustive: bool = switch (item.tag) {
        .type_enum_auto => .{ false, false },
        .type_enum_explicit => .{ true, false },
        .type_enum_nonexhaustive => .{ true, true },
        else => unreachable,
    };
    const extra = extraDataTrail(extra_list, Tag.TypeEnum, item.data);
    var extra_index: u32 = @intCast(extra.end);
    const zir_index: TrackedInst.Index.Optional, const captures: CaptureValue.Slice, const owner_union: Index = switch (extra.data.bits.captures_len) {
        .reified => info: {
            const zir_index: TrackedInst.Index = @enumFromInt(extra_items[extra_index]);
            extra_index += 1;
            extra_index += 2; // type_hash: PackedU64
            break :info .{ zir_index.toOptional(), .empty, .none };
        },
        .generated_union_tag => info: {
            const owner_union: Index = @enumFromInt(extra_items[extra_index]);
            extra_index += 1;
            break :info .{ .none, .empty, owner_union };
        },
        _ => |n| info: {
            const zir_index: TrackedInst.Index = @enumFromInt(extra_items[extra_index]);
            extra_index += 1;
            const captures: CaptureValue.Slice = .{
                .tid = unwrapped_index.tid,
                .start = extra_index,
                .len = @intFromEnum(n),
            };
            extra_index += captures.len;
            break :info .{ zir_index.toOptional(), captures, .none };
        },
    };
    const field_value_map: OptionalMapIndex = if (explicit_int_tag) m: {
        const map: MapIndex = @enumFromInt(extra_items[extra_index]);
        extra_index += 1;
        break :m map.toOptional();
    } else .none;
    const field_names: NullTerminatedString.Slice = .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    };
    extra_index += field_names.len;
    const field_values: Index.Slice = if (explicit_int_tag) .{
        .tid = unwrapped_index.tid,
        .start = extra_index,
        .len = extra.data.fields_len,
    } else .empty;
    extra_index += field_values.len;
    return .{
        .zir_index = zir_index,
        .captures = captures,
        .is_reified = extra.data.bits.captures_len == .reified,
        .owner_union = owner_union,
        .name = extra.data.name,
        .name_nav = extra.data.name_nav,
        .namespace = extra.data.namespace,
        .int_tag_type = extra.data.int_tag_type,
        .int_tag_mode = if (explicit_int_tag) .explicit else .auto,
        .nonexhaustive = nonexhaustive,
        .want_layout = extra.data.bits.want_layout,
        .field_name_map = extra.data.field_name_map,
        .field_value_map = field_value_map,
        .field_names = field_names,
        .field_values = field_values,
    };
}

pub fn loadOpaqueType(ip: *const InternPool, index: Index) LoadedOpaqueType {
    const unwrapped_index = index.unwrap(ip);
    const item = unwrapped_index.getItem(ip);
    assert(item.tag == .type_opaque);
    const extra = extraDataTrail(unwrapped_index.getExtra(ip), Tag.TypeOpaque, item.data);
    return .{
        .zir_index = extra.data.zir_index,
        .captures = .{
            .tid = unwrapped_index.tid,
            .start = extra.end,
            .len = extra.data.captures_len,
        },
        .name = extra.data.name,
        .name_nav = extra.data.name_nav,
        .namespace = extra.data.namespace,
    };
}

pub const Item = struct {
    tag: Tag,
    /// The doc comments on the respective Tag explain how to interpret this.
    data: u32,
};

/// Represents an index into `map`. It represents the canonical index
/// of a `Value` within this `InternPool`. The values are typed.
/// Two values which have the same type can be equality compared simply
/// by checking if their indexes are equal, provided they are both in
/// the same `InternPool`.
/// When adding a tag to this enum, consider adding a corresponding entry to
/// `primitives` in AstGen.zig.
pub const Index = enum(u32) {
    pub const first_type: Index = .u0_type;
    pub const last_type: Index = .empty_tuple_type;
    pub const first_value: Index = .undef;
    pub const last_value: Index = .empty_tuple;

    u0_type,
    i0_type,
    u1_type,
    u8_type,
    i8_type,
    u16_type,
    i16_type,
    u29_type,
    u32_type,
    i32_type,
    u64_type,
    i64_type,
    u80_type,
    u128_type,
    i128_type,
    u256_type,
    usize_type,
    isize_type,
    c_char_type,
    c_short_type,
    c_ushort_type,
    c_int_type,
    c_uint_type,
    c_long_type,
    c_ulong_type,
    c_longlong_type,
    c_ulonglong_type,
    c_longdouble_type,
    f16_type,
    f32_type,
    f64_type,
    f80_type,
    f128_type,
    anyopaque_type,
    bool_type,
    void_type,
    type_type,
    anyerror_type,
    comptime_int_type,
    comptime_float_type,
    noreturn_type,
    anyframe_type,
    null_type,
    undefined_type,
    enum_literal_type,

    ptr_usize_type,
    ptr_const_comptime_int_type,
    manyptr_u8_type,
    manyptr_const_u8_type,
    manyptr_const_u8_sentinel_0_type,
    slice_const_u8_type,
    slice_const_u8_sentinel_0_type,

    manyptr_const_slice_const_u8_type,
    slice_const_slice_const_u8_type,

    optional_type_type,
    manyptr_const_type_type,
    slice_const_type_type,

    vector_8_i8_type,
    vector_16_i8_type,
    vector_32_i8_type,
    vector_64_i8_type,
    vector_1_u8_type,
    vector_2_u8_type,
    vector_4_u8_type,
    vector_8_u8_type,
    vector_16_u8_type,
    vector_32_u8_type,
    vector_64_u8_type,
    vector_2_i16_type,
    vector_4_i16_type,
    vector_8_i16_type,
    vector_16_i16_type,
    vector_32_i16_type,
    vector_4_u16_type,
    vector_8_u16_type,
    vector_16_u16_type,
    vector_32_u16_type,
    vector_2_i32_type,
    vector_4_i32_type,
    vector_8_i32_type,
    vector_16_i32_type,
    vector_4_u32_type,
    vector_8_u32_type,
    vector_16_u32_type,
    vector_2_i64_type,
    vector_4_i64_type,
    vector_8_i64_type,
    vector_2_u64_type,
    vector_4_u64_type,
    vector_8_u64_type,
    vector_1_u128_type,
    vector_2_u128_type,
    vector_1_u256_type,
    vector_4_f16_type,
    vector_8_f16_type,
    vector_16_f16_type,
    vector_32_f16_type,
    vector_2_f32_type,
    vector_4_f32_type,
    vector_8_f32_type,
    vector_16_f32_type,
    vector_2_f64_type,
    vector_4_f64_type,
    vector_8_f64_type,

    optional_noreturn_type,
    anyerror_void_error_union_type,
    /// Used for the inferred error set of inline/comptime function calls.
    adhoc_inferred_error_set_type,
    /// Represents a type which is unknown.
    /// This is used in functions to represent generic parameter/return types, and
    /// during semantic analysis to represent unknown result types (i.e. where AstGen
    /// thought we would have a result type, but we do not).
    generic_poison_type,
    /// `@TypeOf(.{})`; a tuple with zero elements.
    /// This is not the same as `struct {}`, since that is a struct rather than a tuple.
    empty_tuple_type,

    /// `undefined` (untyped)
    undef,
    /// `@as(bool, undefined)`
    undef_bool,
    /// `@as(usize, undefined)`
    undef_usize,
    /// `@as(u1, undefined)`
    undef_u1,
    /// `0` (comptime_int)
    zero,
    /// `@as(usize, 0)`
    zero_usize,
    /// `@as(u1, 0)`
    zero_u1,
    /// `@as(u8, 0)`
    zero_u8,
    /// `1` (comptime_int)
    one,
    /// `@as(usize, 1)`
    one_usize,
    /// `@as(u1, 1)`
    one_u1,
    /// `@as(u8, 1)`
    one_u8,
    /// `@as(u8, 4)`
    four_u8,
    /// `-1` (comptime_int)
    negative_one,
    /// `{}`
    void_value,
    /// `unreachable` (noreturn type)
    unreachable_value,
    /// `null` (untyped)
    null_value,
    /// `true`
    bool_true,
    /// `false`
    bool_false,
    /// `.{}`
    empty_tuple,

    /// Used by Air/Sema only.
    none = std.math.maxInt(u32),

    _,

    /// An array of `Index` existing within the `extra` array.
    /// This type exists to provide a struct with lifetime that is
    /// not invalidated when items are added to the `InternPool`.
    pub const Slice = struct {
        tid: Zcu.PerThread.Id,
        start: u32,
        len: u32,

        pub const empty: Slice = .{ .tid = .main, .start = 0, .len = 0 };

        pub fn get(slice: Slice, ip: *const InternPool) []Index {
            const extra = ip.getLocalShared(slice.tid).extra.acquire();
            return @ptrCast(extra.view().items(.@"0")[slice.start..][0..slice.len]);
        }

        /// If `slice` is empty (`slice.len == 0`), returns `.none`.
        /// Otherwise, asserts that `index < slice.len`, and returns the value at `index`.
        pub fn getOrNone(slice: Slice, ip: *const InternPool, index: usize) Index {
            if (slice.len == 0) return .none;
            return slice.get(ip)[index];
        }
    };

    /// Used for a map of `Index` values to the index within a list of `Index` values.
    const Adapter = struct {
        indexes: []const Index,

        pub fn eql(ctx: @This(), a: Index, b_void: void, b_map_index: usize) bool {
            _ = b_void;
            return a == ctx.indexes[b_map_index];
        }

        pub fn hash(ctx: @This(), a: Index) u32 {
            _ = ctx;
            return std.hash.int(@intFromEnum(a));
        }
    };

    const Unwrapped = struct {
        tid: Zcu.PerThread.Id,
        index: u32,

        fn wrap(unwrapped: Unwrapped, ip: *const InternPool) Index {
            assert(@intFromEnum(unwrapped.tid) <= ip.getTidMask());
            assert(unwrapped.index <= ip.getIndexMask(u30));
            return @enumFromInt(@shlExact(@as(u32, @intFromEnum(unwrapped.tid)), ip.tid_shift_30) |
                unwrapped.index);
        }

        pub fn getExtra(unwrapped: Unwrapped, ip: *const InternPool) Local.Extra {
            return ip.getLocalShared(unwrapped.tid).extra.acquire();
        }

        pub fn getItem(unwrapped: Unwrapped, ip: *const InternPool) Item {
            const item_ptr = unwrapped.itemPtr(ip);
            const tag = @atomicLoad(Tag, item_ptr.tag_ptr, .acquire);
            return .{ .tag = tag, .data = item_ptr.data_ptr.* };
        }

        pub fn getTag(unwrapped: Unwrapped, ip: *const InternPool) Tag {
            const item_ptr = unwrapped.itemPtr(ip);
            return @atomicLoad(Tag, item_ptr.tag_ptr, .acquire);
        }

        pub fn getData(unwrapped: Unwrapped, ip: *const InternPool) u32 {
            return unwrapped.getItem(ip).data;
        }

        const ItemPtr = struct {
            tag_ptr: *Tag,
            data_ptr: *u32,
        };
        fn itemPtr(unwrapped: Unwrapped, ip: *const InternPool) ItemPtr {
            const slice = ip.getLocalShared(unwrapped.tid).items.acquire().view().slice();
            return .{
                .tag_ptr = &slice.items(.tag)[unwrapped.index],
                .data_ptr = &slice.items(.data)[unwrapped.index],
            };
        }

        const debug_state = InternPool.debug_state;
    };
    pub fn unwrap(index: Index, ip: *const InternPool) Unwrapped {
        return if (single_threaded) .{
            .tid = .main,
            .index = @intFromEnum(index),
        } else .{
            .tid = @enumFromInt(@intFromEnum(index) >> ip.tid_shift_30 & ip.getTidMask()),
            .index = @intFromEnum(index) & ip.getIndexMask(u30),
        };
    }

    /// This function is used in the debugger pretty formatters in tools/ to fetch the
    /// Tag to encoding mapping to facilitate fancy debug printing for this type.
    fn dbHelper(self: *Index, tag_to_encoding_map: *struct {
        const DataIsIndex = struct { data: Index };

        removed: void,
        type_int_signed: struct { data: u32 },
        type_int_unsigned: struct { data: u32 },
        type_array_big: struct { data: *Array },
        type_array_small: struct { data: *Vector },
        type_vector: struct { data: *Vector },
        type_pointer: struct { data: *Tag.TypePointer },
        type_slice: DataIsIndex,
        type_optional: DataIsIndex,
        type_anyframe: DataIsIndex,
        type_error_union: struct { data: *Key.ErrorUnionType },
        type_anyerror_union: DataIsIndex,
        type_error_set: struct {
            const @"data.names_len" = opaque {};
            data: *Tag.ErrorSet,
            @"trailing.names.len": *@"data.names_len",
            trailing: struct { names: []NullTerminatedString },
        },
        type_inferred_error_set: DataIsIndex,
        simple_type: void,
        type_function: struct {
            const @"data.flags.has_comptime_bits" = opaque {};
            const @"data.flags.has_noalias_bits" = opaque {};
            const @"data.params_len" = opaque {};
            data: *Tag.TypeFunction,
            @"trailing.comptime_bits.len": *@"data.flags.has_comptime_bits",
            @"trailing.noalias_bits.len": *@"data.flags.has_noalias_bits",
            @"trailing.param_types.len": *@"data.params_len",
            trailing: struct { comptime_bits: []u32, noalias_bits: []u32, param_types: []Index },
        },
        type_tuple: struct {
            const @"data.fields_len" = opaque {};
            data: *TypeTuple,
            @"trailing.types.len": *@"data.fields_len",
            @"trailing.values.len": *@"data.fields_len",
            trailing: struct {
                types: []Index,
                values: []Index,
            },
        },

        type_struct: struct { data: *Tag.TypeStruct },
        type_struct_packed_auto: struct { data: *Tag.TypeStructPacked },
        type_struct_packed_explicit: struct { data: *Tag.TypeStructPacked },
        type_struct_packed_auto_defaults: struct { data: *Tag.TypeStructPacked },
        type_struct_packed_explicit_defaults: struct { data: *Tag.TypeStructPacked },
        type_union: struct { data: *Tag.TypeUnion },
        type_union_packed_auto: struct { data: *Tag.TypeUnionPacked },
        type_union_packed_explicit: struct { data: *Tag.TypeUnionPacked },
        type_enum_auto: struct { data: *Tag.TypeEnum },
        type_enum_explicit: struct { data: *Tag.TypeEnum },
        type_enum_nonexhaustive: struct { data: *Tag.TypeEnum },
        type_opaque: struct { data: *Tag.TypeOpaque },

        undef: DataIsIndex,
        simple_value: void,
        ptr_nav: struct { data: *PtrNav },
        ptr_comptime_alloc: struct { data: *PtrComptimeAlloc },
        ptr_uav: struct { data: *PtrUav },
        ptr_uav_aligned: struct { data: *PtrUavAligned },
        ptr_comptime_field: struct { data: *PtrComptimeField },
        ptr_int: struct { data: *PtrInt },
        ptr_eu_payload: struct { data: *PtrBase },
        ptr_opt_payload: struct { data: *PtrBase },
        ptr_elem: struct { data: *PtrBaseIndex },
        ptr_field: struct { data: *PtrBaseIndex },
        ptr_slice: struct { data: *PtrSlice },
        opt_payload: struct { data: *Tag.TypeValue },
        opt_null: DataIsIndex,
        int_u8: struct { data: u8 },
        int_u16: struct { data: u16 },
        int_u32: struct { data: u32 },
        int_i32: struct { data: i32 },
        int_usize: struct { data: u32 },
        int_comptime_int_u32: struct { data: u32 },
        int_comptime_int_i32: struct { data: i32 },
        int_small: struct { data: *IntSmall },
        int_positive: struct { data: u32 },
        int_negative: struct { data: u32 },
        error_set_error: struct { data: *Key.Error },
        error_union_error: struct { data: *Key.Error },
        error_union_payload: struct { data: *Tag.TypeValue },
        enum_literal: struct { data: NullTerminatedString },
        enum_tag: struct { data: *Tag.EnumTag },
        float_f16: struct { data: f16 },
        float_f32: struct { data: f32 },
        float_f64: struct { data: *Float64 },
        float_f80: struct { data: *Float80 },
        float_f128: struct { data: *Float128 },
        float_c_longdouble_f80: struct { data: *Float80 },
        float_c_longdouble_f128: struct { data: *Float128 },
        float_comptime_float: struct { data: *Float128 },
        variable: struct { data: *Tag.Variable },
        threadlocal_variable: struct { data: *Tag.Variable },
        @"extern": struct { data: *Tag.Extern },
        func_decl: struct {
            const @"data.analysis.inferred_error_set" = opaque {};
            data: *Tag.FuncDecl,
            @"trailing.resolved_error_set.len": *@"data.analysis.inferred_error_set",
            trailing: struct { resolved_error_set: []Index },
        },
        func_instance: struct {
            const @"data.analysis.inferred_error_set" = opaque {};
            const @"data.generic_owner.data.ty.data.params_len" = opaque {};
            data: *Tag.FuncInstance,
            @"trailing.resolved_error_set.len": *@"data.analysis.inferred_error_set",
            @"trailing.comptime_args.len": *@"data.generic_owner.data.ty.data.params_len",
            trailing: struct { resolved_error_set: []Index, comptime_args: []Index },
        },
        func_coerced: struct {
            data: *Tag.FuncCoerced,
        },
        only_possible_value: DataIsIndex,
        union_value: struct { data: *Key.Union },
        bytes: struct { data: *Bytes },
        aggregate: struct {
            const @"data.ty.data.len orelse data.ty.data.fields_len" = opaque {};
            data: *Tag.Aggregate,
            @"trailing.element_values.len": *@"data.ty.data.len orelse data.ty.data.fields_len",
            trailing: struct { element_values: []Index },
        },
        repeated: struct { data: *Repeated },
        bitpack: struct { data: *Key.Bitpack },

        memoized_call: struct {
            const @"data.args_len" = opaque {};
            data: *MemoizedCall,
            @"trailing.arg_values.len": *@"data.args_len",
            trailing: struct { arg_values: []Index },
        },
    }) void {
        _ = self;
        const map_fields = @typeInfo(@typeInfo(@TypeOf(tag_to_encoding_map)).pointer.child).@"struct".fields;
        @setEvalBranchQuota(3_000);
        inline for (@typeInfo(Tag).@"enum".fields, 0..) |tag, start| {
            inline for (0..map_fields.len) |offset| {
                if (comptime std.mem.eql(u8, tag.name, map_fields[(start + offset) % map_fields.len].name)) break;
            } else {
                @compileError(@typeName(Tag) ++ "." ++ tag.name ++ " missing dbHelper tag_to_encoding_map entry");
            }
        }
    }
    comptime {
        if (!builtin.strip_debug_info) switch (builtin.zig_backend) {
            .stage2_llvm => _ = &dbHelper,
            .stage2_x86_64 => for (@typeInfo(Tag).@"enum".fields) |tag| {
                if (!@hasField(@TypeOf(Tag.encodings), tag.name)) @compileLog("missing: " ++ @typeName(Tag) ++ ".encodings." ++ tag.name);
                const encoding = @field(Tag.encodings, tag.name);
                if (@hasField(@TypeOf(encoding), "trailing")) for (@typeInfo(encoding.trailing).@"struct".fields) |field| {
                    struct {
                        fn checkConfig(name: []const u8) void {
                            if (!@hasField(@TypeOf(encoding.config), name)) @compileError("missing field: " ++ @typeName(Tag) ++ ".encodings." ++ tag.name ++ ".config.@\"" ++ name ++ "\"");
                            const FieldType = @TypeOf(@field(encoding.config, name));
                            if (@typeInfo(FieldType) != .enum_literal) @compileError("expected enum literal: " ++ @typeName(Tag) ++ ".encodings." ++ tag.name ++ ".config.@\"" ++ name ++ "\": " ++ @typeName(FieldType));
                        }
                        fn checkField(name: []const u8, Type: type) void {
                            switch (@typeInfo(Type)) {
                                .int => {},
                                .@"enum" => {},
                                .@"struct" => |info| assert(info.layout == .@"packed"),
                                .optional => |info| {
                                    checkConfig(name ++ ".?");
                                    checkField(name ++ ".?", info.child);
                                },
                                .pointer => |info| {
                                    assert(info.size == .slice);
                                    checkConfig(name ++ ".len");
                                    checkField(name ++ "[0]", info.child);
                                },
                                else => @compileError("unsupported type: " ++ @typeName(Tag) ++ ".encodings." ++ tag.name ++ "." ++ name ++ ": " ++ @typeName(Type)),
                            }
                        }
                    }.checkField("trailing." ++ field.name, field.type);
                };
            },
            else => {},
        };
    }
};

pub const static_keys: [static_len]Key = .{
    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 0,
    } },

    .{ .int_type = .{
        .signedness = .signed,
        .bits = 0,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 1,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 8,
    } },

    .{ .int_type = .{
        .signedness = .signed,
        .bits = 8,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 16,
    } },

    .{ .int_type = .{
        .signedness = .signed,
        .bits = 16,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 29,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 32,
    } },

    .{ .int_type = .{
        .signedness = .signed,
        .bits = 32,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 64,
    } },

    .{ .int_type = .{
        .signedness = .signed,
        .bits = 64,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 80,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 128,
    } },

    .{ .int_type = .{
        .signedness = .signed,
        .bits = 128,
    } },

    .{ .int_type = .{
        .signedness = .unsigned,
        .bits = 256,
    } },

    .{ .simple_type = .usize },
    .{ .simple_type = .isize },
    .{ .simple_type = .c_char },
    .{ .simple_type = .c_short },
    .{ .simple_type = .c_ushort },
    .{ .simple_type = .c_int },
    .{ .simple_type = .c_uint },
    .{ .simple_type = .c_long },
    .{ .simple_type = .c_ulong },
    .{ .simple_type = .c_longlong },
    .{ .simple_type = .c_ulonglong },
    .{ .simple_type = .c_longdouble },
    .{ .simple_type = .f16 },
    .{ .simple_type = .f32 },
    .{ .simple_type = .f64 },
    .{ .simple_type = .f80 },
    .{ .simple_type = .f128 },
    .{ .simple_type = .anyopaque },
    .{ .simple_type = .bool },
    .{ .simple_type = .void },
    .{ .simple_type = .type },
    .{ .simple_type = .anyerror },
    .{ .simple_type = .comptime_int },
    .{ .simple_type = .comptime_float },
    .{ .simple_type = .noreturn },
    .{ .anyframe_type = .none },
    .{ .simple_type = .null },
    .{ .simple_type = .undefined },
    .{ .simple_type = .enum_literal },

    // *usize
    .{ .ptr_type = .{
        .child = .usize_type,
        .flags = .{},
    } },

    // *const comptime_int
    .{ .ptr_type = .{
        .child = .comptime_int_type,
        .flags = .{
            .is_const = true,
        },
    } },

    // [*]u8
    .{ .ptr_type = .{
        .child = .u8_type,
        .flags = .{
            .size = .many,
        },
    } },

    // [*]const u8
    .{ .ptr_type = .{
        .child = .u8_type,
        .flags = .{
            .size = .many,
            .is_const = true,
        },
    } },

    // [*:0]const u8
    .{ .ptr_type = .{
        .child = .u8_type,
        .sentinel = .zero_u8,
        .flags = .{
            .size = .many,
            .is_const = true,
        },
    } },

    // []const u8
    .{ .ptr_type = .{
        .child = .u8_type,
        .flags = .{
            .size = .slice,
            .is_const = true,
        },
    } },

    // [:0]const u8
    .{ .ptr_type = .{
        .child = .u8_type,
        .sentinel = .zero_u8,
        .flags = .{
            .size = .slice,
            .is_const = true,
        },
    } },

    // [*]const []const u8
    .{ .ptr_type = .{
        .child = .slice_const_u8_type,
        .flags = .{
            .size = .many,
            .is_const = true,
        },
    } },

    // []const []const u8
    .{ .ptr_type = .{
        .child = .slice_const_u8_type,
        .flags = .{
            .size = .slice,
            .is_const = true,
        },
    } },

    // ?type
    .{ .opt_type = .type_type },

    // [*]const type
    .{ .ptr_type = .{
        .child = .type_type,
        .flags = .{
            .size = .many,
            .is_const = true,
        },
    } },

    // []const type
    .{ .ptr_type = .{
        .child = .type_type,
        .flags = .{
            .size = .slice,
            .is_const = true,
        },
    } },

    // @Vector(8, i8)
    .{ .vector_type = .{ .len = 8, .child = .i8_type } },
    // @Vector(16, i8)
    .{ .vector_type = .{ .len = 16, .child = .i8_type } },
    // @Vector(32, i8)
    .{ .vector_type = .{ .len = 32, .child = .i8_type } },
    // @Vector(64, i8)
    .{ .vector_type = .{ .len = 64, .child = .i8_type } },
    // @Vector(1, u8)
    .{ .vector_type = .{ .len = 1, .child = .u8_type } },
    // @Vector(2, u8)
    .{ .vector_type = .{ .len = 2, .child = .u8_type } },
    // @Vector(4, u8)
    .{ .vector_type = .{ .len = 4, .child = .u8_type } },
    // @Vector(8, u8)
    .{ .vector_type = .{ .len = 8, .child = .u8_type } },
    // @Vector(16, u8)
    .{ .vector_type = .{ .len = 16, .child = .u8_type } },
    // @Vector(32, u8)
    .{ .vector_type = .{ .len = 32, .child = .u8_type } },
    // @Vector(64, u8)
    .{ .vector_type = .{ .len = 64, .child = .u8_type } },
    // @Vector(2, i16)
    .{ .vector_type = .{ .len = 2, .child = .i16_type } },
    // @Vector(4, i16)
    .{ .vector_type = .{ .len = 4, .child = .i16_type } },
    // @Vector(8, i16)
    .{ .vector_type = .{ .len = 8, .child = .i16_type } },
    // @Vector(16, i16)
    .{ .vector_type = .{ .len = 16, .child = .i16_type } },
    // @Vector(32, i16)
    .{ .vector_type = .{ .len = 32, .child = .i16_type } },
    // @Vector(4, u16)
    .{ .vector_type = .{ .len = 4, .child = .u16_type } },
    // @Vector(8, u16)
    .{ .vector_type = .{ .len = 8, .child = .u16_type } },
    // @Vector(16, u16)
    .{ .vector_type = .{ .len = 16, .child = .u16_type } },
    // @Vector(32, u16)
    .{ .vector_type = .{ .len = 32, .child = .u16_type } },
    // @Vector(2, i32)
    .{ .vector_type = .{ .len = 2, .child = .i32_type } },
    // @Vector(4, i32)
    .{ .vector_type = .{ .len = 4, .child = .i32_type } },
    // @Vector(8, i32)
    .{ .vector_type = .{ .len = 8, .child = .i32_type } },
    // @Vector(16, i32)
    .{ .vector_type = .{ .len = 16, .child = .i32_type } },
    // @Vector(4, u32)
    .{ .vector_type = .{ .len = 4, .child = .u32_type } },
    // @Vector(8, u32)
    .{ .vector_type = .{ .len = 8, .child = .u32_type } },
    // @Vector(16, u32)
    .{ .vector_type = .{ .len = 16, .child = .u32_type } },
    // @Vector(2, i64)
    .{ .vector_type = .{ .len = 2, .child = .i64_type } },
    // @Vector(4, i64)
    .{ .vector_type = .{ .len = 4, .child = .i64_type } },
    // @Vector(8, i64)
    .{ .vector_type = .{ .len = 8, .child = .i64_type } },
    // @Vector(2, u64)
    .{ .vector_type = .{ .len = 2, .child = .u64_type } },
    // @Vector(4, u64)
    .{ .vector_type = .{ .len = 4, .child = .u64_type } },
    // @Vector(8, u64)
    .{ .vector_type = .{ .len = 8, .child = .u64_type } },
    // @Vector(1, u128)
    .{ .vector_type = .{ .len = 1, .child = .u128_type } },
    // @Vector(2, u128)
    .{ .vector_type = .{ .len = 2, .child = .u128_type } },
    // @Vector(1, u256)
    .{ .vector_type = .{ .len = 1, .child = .u256_type } },
    // @Vector(4, f16)
    .{ .vector_type = .{ .len = 4, .child = .f16_type } },
    // @Vector(8, f16)
    .{ .vector_type = .{ .len = 8, .child = .f16_type } },
    // @Vector(16, f16)
    .{ .vector_type = .{ .len = 16, .child = .f16_type } },
    // @Vector(32, f16)
    .{ .vector_type = .{ .len = 32, .child = .f16_type } },
    // @Vector(2, f32)
    .{ .vector_type = .{ .len = 2, .child = .f32_type } },
    // @Vector(4, f32)
    .{ .vector_type = .{ .len = 4, .child = .f32_type } },
    // @Vector(8, f32)
    .{ .vector_type = .{ .len = 8, .child = .f32_type } },
    // @Vector(16, f32)
    .{ .vector_type = .{ .len = 16, .child = .f32_type } },
    // @Vector(2, f64)
    .{ .vector_type = .{ .len = 2, .child = .f64_type } },
    // @Vector(4, f64)
    .{ .vector_type = .{ .len = 4, .child = .f64_type } },
    // @Vector(8, f64)
    .{ .vector_type = .{ .len = 8, .child = .f64_type } },

    // ?noreturn
    .{ .opt_type = .noreturn_type },

    // anyerror!void
    .{ .error_union_type = .{
        .error_set_type = .anyerror_type,
        .payload_type = .void_type,
    } },

    // adhoc_inferred_error_set_type
    .{ .simple_type = .adhoc_inferred_error_set },
    // generic_poison_type
    .{ .simple_type = .generic_poison },

    // empty_tuple_type
    .{ .tuple_type = .{
        .types = .empty,
        .values = .empty,
    } },

    .{ .undef = .undefined_type },
    .{ .undef = .bool_type },
    .{ .undef = .usize_type },
    .{ .undef = .u1_type },

    .{ .int = .{
        .ty = .comptime_int_type,
        .storage = .{ .u64 = 0 },
    } },

    .{ .int = .{
        .ty = .usize_type,
        .storage = .{ .u64 = 0 },
    } },

    .{ .int = .{
        .ty = .u1_type,
        .storage = .{ .u64 = 0 },
    } },

    .{ .int = .{
        .ty = .u8_type,
        .storage = .{ .u64 = 0 },
    } },

    .{ .int = .{
        .ty = .comptime_int_type,
        .storage = .{ .u64 = 1 },
    } },

    .{ .int = .{
        .ty = .usize_type,
        .storage = .{ .u64 = 1 },
    } },

    .{ .int = .{
        .ty = .u1_type,
        .storage = .{ .u64 = 1 },
    } },

    .{ .int = .{
        .ty = .u8_type,
        .storage = .{ .u64 = 1 },
    } },

    .{ .int = .{
        .ty = .u8_type,
        .storage = .{ .u64 = 4 },
    } },

    .{ .int = .{
        .ty = .comptime_int_type,
        .storage = .{ .i64 = -1 },
    } },

    .{ .simple_value = .void },
    .{ .simple_value = .@"unreachable" },
    .{ .simple_value = .null },
    .{ .simple_value = .true },
    .{ .simple_value = .false },

    .{ .aggregate = .{
        .ty = .empty_tuple_type,
        .storage = .{ .elems = &.{} },
    } },
};

/// How many items in the InternPool are statically known.
/// This is specified with an integer literal and a corresponding comptime
/// assert below to break an unfortunate and arguably incorrect dependency loop
/// when compiling.
pub const static_len = Zir.Inst.Ref.static_len;

pub const Tag = enum(u8) {
    /// This special tag represents a value which was removed from this pool via
    /// `InternPool.remove`. The item remains allocated to preserve indices, but
    /// lookups will consider it not equal to any other item, and all queries
    /// assert not this tag. `data` is unused.
    removed,

    /// A type that can be represented with only an enum tag.
    simple_type,
    /// An integer type.
    /// data is number of bits
    type_int_signed,
    /// An integer type.
    /// data is number of bits
    type_int_unsigned,
    /// An array type whose length requires 64 bits or which has a sentinel.
    /// data is payload to Array.
    type_array_big,
    /// An array type that has no sentinel and whose length fits in 32 bits.
    /// data is payload to Vector.
    type_array_small,
    /// A vector type.
    /// data is payload to Vector.
    type_vector,
    /// A fully explicitly specified pointer type.
    type_pointer,
    /// A slice type.
    /// data is Index of underlying pointer type.
    type_slice,
    /// An optional type.
    /// data is the child type.
    type_optional,
    /// The type `anyframe->T`.
    /// data is the child type.
    /// If the child type is `none`, the type is `anyframe`.
    type_anyframe,
    /// An error union type.
    /// data is payload to `Key.ErrorUnionType`.
    type_error_union,
    /// An error union type of the form `anyerror!T`.
    /// data is `Index` of payload type.
    type_anyerror_union,
    /// An error set type.
    /// data is payload to `ErrorSet`.
    type_error_set,
    /// The inferred error set type of a function.
    /// data is `Index` of a `func_decl` or `func_instance`.
    type_inferred_error_set,
    /// A function body type.
    /// `data` is extra index to `TypeFunction`.
    type_function,
    /// A `TupleType`.
    /// data is extra index of `TypeTuple`.
    type_tuple,

    /// A non-packed struct type.
    /// data is extra index of `TypeStruct`.
    type_struct,
    /// `packed struct { ... }` with no default field values.
    /// data is extra index of `TypeStructPacked`.
    type_struct_packed_auto,
    /// `packed struct(T) { ... }` with no default field values.
    /// data is extra index of `TypeStructPacked`.
    type_struct_packed_explicit,
    /// `packed struct { ... }` with one or more default field values.
    /// data is extra index of `TypeStructPacked`.
    type_struct_packed_auto_defaults,
    /// `packed struct(T) { ... }` with one or more default field values.
    /// data is extra index of `TypeStructPacked`.
    type_struct_packed_explicit_defaults,

    /// A non-packed union type.
    /// data is extra index of `TypeUnion`.
    type_union,
    /// `packed union { ... }`.
    /// data is extra index of `TypeUnionPacked`.
    type_union_packed_auto,
    /// `packed union(T) { ... }`.
    /// data is extra index of `TypeUnionPacked`.
    type_union_packed_explicit,

    /// An exhaustive enum type *without* an explicit integer tag type. The tag type is inferred.
    ///
    /// Because the tag type is inferred, there are no explicit field values.
    ///
    /// May be the generated tag type for a `union(enum)`.
    ///
    /// data is extra index of `TypeEnum`.
    type_enum_auto,
    /// An exhaustive enum type *with* an explicit integer tag type.
    ///
    /// May have explicit field values.
    ///
    /// May be the generated tag type for a `union(enum(T))`.
    ///
    /// data is extra index of `TypeEnum`.
    type_enum_explicit,
    /// An non-exhaustive enum type (with an explicit integer tag type, since it is required for
    /// non-exhaustive enums).
    ///
    /// May have explicit field values.
    ///
    /// This is *not* a union's generated tag type, because such types are always exhaustive.
    ///
    /// data is extra index of `TypeEnum`.
    type_enum_nonexhaustive,

    /// An opaque type.
    /// data is extra index of `TypeOpaque`.
    type_opaque,

    /// Typed `undefined`.
    /// `data` is `Index` of the type.
    /// Untyped `undefined` is stored instead via `simple_value`.
    undef,
    /// A value that can be represented with only an enum tag.
    simple_value,
    /// A pointer to a `Nav`.
    /// data is extra index of `PtrNav`, which contains the type and address.
    ptr_nav,
    /// A pointer to a decl that can be mutated at comptime.
    /// data is extra index of `PtrComptimeAlloc`, which contains the type and address.
    ptr_comptime_alloc,
    /// A pointer to an anonymous addressable value.
    /// data is extra index of `PtrUav`, which contains the pointer type and decl value.
    /// The alignment of the uav is communicated via the pointer type.
    ptr_uav,
    /// A pointer to an unnamed addressable value.
    /// data is extra index of `PtrUavAligned`, which contains the pointer
    /// type and decl value.
    /// The original pointer type is also provided, which will be different than `ty`.
    /// This encoding is only used when a pointer to a Uav is
    /// coerced to a different pointer type with a different alignment.
    ptr_uav_aligned,
    /// data is extra index of `PtrComptimeField`, which contains the pointer type and field value.
    ptr_comptime_field,
    /// A pointer with an integer value.
    /// data is extra index of `PtrInt`, which contains the type and address (byte offset from 0).
    /// Only pointer types are allowed to have this encoding. Optional types must use
    /// `opt_payload` or `opt_null`.
    ptr_int,
    /// A pointer to the payload of an error union.
    /// data is extra index of `PtrBase`, which contains the type and base pointer.
    ptr_eu_payload,
    /// A pointer to the payload of an optional.
    /// data is extra index of `PtrBase`, which contains the type and base pointer.
    ptr_opt_payload,
    /// A pointer to an array element.
    /// data is extra index of PtrBaseIndex, which contains the base array and element index.
    /// In order to use this encoding, one must ensure that the `InternPool`
    /// already contains the elem pointer type corresponding to this payload.
    ptr_elem,
    /// A pointer to a container field.
    /// data is extra index of PtrBaseIndex, which contains the base container and field index.
    ptr_field,
    /// A slice.
    /// data is extra index of PtrSlice, which contains the ptr and len values
    ptr_slice,
    /// An optional value that is non-null.
    /// data is extra index of `TypeValue`.
    /// The type is the optional type (not the payload type).
    opt_payload,
    /// An optional value that is null.
    /// data is Index of the optional type.
    opt_null,
    /// Type: u8
    /// data is integer value
    int_u8,
    /// Type: u16
    /// data is integer value
    int_u16,
    /// Type: u32
    /// data is integer value
    int_u32,
    /// Type: i32
    /// data is integer value bitcasted to u32.
    int_i32,
    /// A usize that fits in 32 bits.
    /// data is integer value.
    int_usize,
    /// A comptime_int that fits in a u32.
    /// data is integer value.
    int_comptime_int_u32,
    /// A comptime_int that fits in an i32.
    /// data is integer value bitcasted to u32.
    int_comptime_int_i32,
    /// An integer value that fits in 32 bits with an explicitly provided type.
    /// data is extra index of `IntSmall`.
    int_small,
    /// A positive integer value.
    /// data is a limbs index to `Int`.
    int_positive,
    /// A negative integer value.
    /// data is a limbs index to `Int`.
    int_negative,
    /// An error value.
    /// data is extra index of `Key.Error`.
    error_set_error,
    /// An error union error.
    /// data is extra index of `Key.Error`.
    error_union_error,
    /// An error union payload.
    /// data is extra index of `TypeValue`.
    error_union_payload,
    /// An enum literal value.
    /// data is `NullTerminatedString` of the error name.
    enum_literal,
    /// An enum tag value.
    /// data is extra index of `EnumTag`.
    enum_tag,
    /// An f16 value.
    /// data is float value bitcasted to u16 and zero-extended.
    float_f16,
    /// An f32 value.
    /// data is float value bitcasted to u32.
    float_f32,
    /// An f64 value.
    /// data is extra index to Float64.
    float_f64,
    /// An f80 value.
    /// data is extra index to Float80.
    float_f80,
    /// An f128 value.
    /// data is extra index to Float128.
    float_f128,
    /// A c_longdouble value of 80 bits.
    /// data is extra index to Float80.
    /// This is used when a c_longdouble value is provided as an f80, because f80 has unnormalized
    /// values which cannot be losslessly represented as f128. It should only be used when the type
    /// underlying c_longdouble for the target is 80 bits.
    float_c_longdouble_f80,
    /// A c_longdouble value of 128 bits.
    /// data is extra index to Float128.
    /// This is used when a c_longdouble value is provided as any type other than an f80, since all
    /// other float types can be losslessly converted to and from f128.
    float_c_longdouble_f128,
    /// A comptime_float value.
    /// data is extra index to Float128.
    float_comptime_float,
    /// A global variable.
    /// data is extra index to Variable.
    variable,
    /// A global threadlocal variable.
    /// data is extra index to Variable.
    threadlocal_variable,
    /// An extern function or variable.
    /// data is extra index to Extern.
    /// Some parts of the key are stored in `owner_nav`.
    @"extern",
    /// A non-extern function corresponding directly to the AST node from whence it originated.
    /// data is extra index to `FuncDecl`.
    /// Only the owner Decl is used for hashing and equality because the other
    /// fields can get patched up during incremental compilation.
    func_decl,
    /// A generic function instantiation.
    /// data is extra index to `FuncInstance`.
    func_instance,
    /// A `func_decl` or a `func_instance` that has been coerced to a different type.
    /// data is extra index to `FuncCoerced`.
    func_coerced,
    /// This represents the only possible value for *some* types which have
    /// only one possible value. Not all only-possible-values are encoded this way;
    /// for example structs which have all comptime fields are not encoded this way.
    /// The set of values that are encoded this way is:
    /// * An array or vector which has length 0.
    /// * A struct which has all fields comptime-known.
    /// * An empty enum or union. TODO: this value's existence is strange, because such a type in reality has no values. See #15909
    /// data is Index of the type, which is known to be zero bits at runtime.
    only_possible_value,
    /// data is extra index to Key.Union.
    union_value,
    /// An array of bytes.
    /// data is extra index to `Bytes`.
    bytes,
    /// An instance of a struct, array, or vector.
    /// data is extra index to `Aggregate`.
    aggregate,
    /// An instance of an array or vector with every element being the same value.
    /// data is extra index to `Repeated`.
    repeated,
    /// An instance of a `packed struct` or `packed union`.
    /// data is extra index to `Key.Bitpack`.
    bitpack,

    /// A memoized comptime function call result.
    /// data is extra index to `MemoizedCall`
    memoized_call,

    const ErrorUnionType = Key.ErrorUnionType;
    const TypeValue = Key.TypeValue;
    const Error = Key.Error;
    const EnumTag = Key.EnumTag;
    const Union = Key.Union;
    const TypePointer = Key.PtrType;

    const struct_packed_encoding = .{
        .summary = .@"{.payload.name%summary#\"}",
        .payload = TypeStructPacked,
        .trailing = struct {
            type_hash: ?u64,
            captures: ?[]CaptureValue,
            field_names: []NullTerminatedString,
            field_types: []Index,
        },
        .config = .{
            .@"trailing.type_hash.?" = .@"payload.captures_len == .reified",
            .@"trailing.captures.?" = .@"payload.captures_len != .reified",
            .@"trailing.captures.?.len" = .@"@intFromEnum(payload.captures_len)",
            .@"trailing.field_names.len" = .@"payload.fields_len",
            .@"trailing.field_types.len" = .@"payload.fields_len",
        },
    };
    const struct_packed_defaults_encoding = .{
        .summary = .@"{.payload.name%summary#\"}",
        .payload = TypeStructPacked,
        .trailing = struct {
            type_hash: ?u64,
            captures: ?[]CaptureValue,
            field_names: []NullTerminatedString,
            field_types: []Index,
            field_defaults: []Index,
        },
        .config = .{
            .@"trailing.type_hash.?" = .@"payload.captures_len == .reified",
            .@"trailing.captures.?" = .@"payload.captures_len != .reified",
            .@"trailing.captures.?.len" = .@"@intFromEnum(payload.captures_len)",
            .@"trailing.field_names.len" = .@"payload.fields_len",
            .@"trailing.field_types.len" = .@"payload.fields_len",
            .@"trailing.field_defaults.len" = .@"payload.fields_len",
        },
    };
    const union_packed_encoding = .{
        .summary = .@"{.payload.name%summary#\"}",
        .payload = TypeUnionPacked,
        .trailing = struct {
            type_hash: ?u64,
            captures: ?[]CaptureValue,
            field_types: []Index,
        },
        .config = .{
            .@"trailing.type_hash.?" = .@"payload.captures_len == .reified",
            .@"trailing.captures.?" = .@"payload.captures_len != .reified",
            .@"trailing.captures.?.len" = .@"@intFromEnum(payload.captures_len)",
            .@"trailing.field_types.len" = .@"payload.fields_len",
        },
    };
    const enum_explicit_encoding = .{
        .summary = .@"{.payload.name%summary#\"}",
        .payload = TypeEnum,
        .trailing = struct {
            owner_union: ?Index,
            zir_index: ?TrackedInst.Index,
            type_hash: ?u64,
            captures: ?[]CaptureValue,
            field_value_map: MapIndex,
            field_names: []NullTerminatedString,
            field_values: []Index,
        },
        .config = .{
            .@"trailing.owner_union.?" = .@"payload.captures_len == .generated_union_tag",
            .@"trailing.zir_index.?" = .@"payload.captures_len != .generated_union_tag",
            .@"trailing.type_hash.?" = .@"payload.captures_len == .reified",
            .@"trailing.captures.?" = .@"payload.captures_len != .reified and payload.captures_len != .generated_enum_tag",
            .@"trailing.captures.?.len" = .@"@intFromEnum(payload.captures_len)",
            .@"trailing.field_names.len" = .@"payload.fields_len",
            .@"trailing.field_values.len" = .@"payload.fields_len",
        },
    };
    const encodings = .{
        .removed = .{},

        .type_int_signed = .{ .summary = .@"i{.data%value}", .data = u32 },
        .type_int_unsigned = .{ .summary = .@"u{.data%value}", .data = u32 },
        .type_array_big = .{
            .summary = .@"[{.payload.len1%value} << 32 | {.payload.len0%value}:{.payload.sentinel%summary}]{.payload.child%summary}",
            .payload = Array,
        },
        .type_array_small = .{ .summary = .@"[{.payload.len%value}]{.payload.child%summary}", .payload = Vector },
        .type_vector = .{ .summary = .@"@Vector({.payload.len%value}, {.payload.child%summary})", .payload = Vector },
        .type_pointer = .{ .summary = .@"*... {.payload.child%summary}", .payload = TypePointer },
        .type_slice = .{ .summary = .@"[]... {.data.unwrapped.payload.child%summary}", .data = Index },
        .type_optional = .{ .summary = .@"?{.data%summary}", .data = Index },
        .type_anyframe = .{ .summary = .@"anyframe->{.data%summary}", .data = Index },
        .type_error_union = .{
            .summary = .@"{.payload.error_set_type%summary}!{.payload.payload_type%summary}",
            .payload = ErrorUnionType,
        },
        .type_anyerror_union = .{ .summary = .@"anyerror!{.data%summary}", .data = Index },
        .type_error_set = .{ .summary = .@"error{...}", .payload = ErrorSet },
        .type_inferred_error_set = .{
            .summary = .@"@typeInfo(@typeInfo(@TypeOf({.data%summary})).@\"fn\".return_type.?).error_union.error_set",
            .data = Index,
        },
        .simple_type = .{ .summary = .@"{.index%value#.}", .index = SimpleType },
        .type_tuple = .{
            .summary = .@"struct {...}",
            .payload = TypeTuple,
            .trailing = struct {
                field_types: []Index,
                field_values: []Index,
            },
            .config = .{
                .@"trailing.field_types.len" = .@"payload.fields_len",
                .@"trailing.field_values.len" = .@"payload.fields_len",
            },
        },
        .type_function = .{
            .summary = .@"fn (...) ... {.payload.return_type%summary}",
            .payload = TypeFunction,
            .trailing = struct {
                param_comptime_bits: ?[]u32,
                param_noalias_bits: ?[]u32,
                param_type: []Index,
            },
            .config = .{
                .@"trailing.param_comptime_bits.?" = .@"payload.flags.has_comptime_bits",
                .@"trailing.param_comptime_bits.?.len" = .@"(payload.params_len + 31) / 32",
                .@"trailing.param_noalias_bits.?" = .@"payload.flags.has_noalias_bits",
                .@"trailing.param_noalias_bits.?.len" = .@"(payload.params_len + 31) / 32",
                .@"trailing.param_type.len" = .@"payload.params_len",
            },
        },

        .type_struct = .{
            .summary = .@"{.payload.name%summary#\"}",
            .payload = TypeStruct,
            .trailing = struct {
                type_hash: ?u64,
                captures_len: ?u32,
                captures: ?[]CaptureValue,
                field_names: []NullTerminatedString,
                field_types: []Index,
                field_defaults: ?[]Index,
                field_aligns: ?[]Alignment,
                field_is_comptime_bits: ?[]u32,
                field_runtime_order: ?[]u32,
                field_offsets: []u32,
            },
            .config = .{
                .@"trailing.type_hash.?" = .@"payload.flags.any_captures == .reified",
                .@"trailing.captures_len.?" = .@"payload.flags.any_captures == .true",
                .@"trailing.captures.?" = .@"payload.flags.any_captures == .true",
                .@"trailing.captures.?.len" = .@"trailing.captures_len.?",
                .@"trailing.field_names.len" = .@"payload.fields_len",
                .@"trailing.field_types.len" = .@"payload.fields_len",
                .@"trailing.field_defaults.?" = .@"payload.flags.any_field_defaults",
                .@"trailing.field_defaults.?.len" = .@"payload.fields_len",
                .@"trailing.field_aligns.?" = .@"payload.flags.any_field_aligns",
                .@"trailing.field_aligns.?.len" = .@"payload.fields_len",
                .@"trailing.field_is_comptime_bits.?" = .@"payload.flags.any_comptime_fields",
                .@"trailing.field_is_comptime_bits.?.len" = .@"(payload.fields_len + 31) / 32",
                .@"trailing.field_runtime_order.?" = .@"payload.flags.layout == .auto",
                .@"trailing.field_runtime_order.?.len" = .@"payload.fields_len",
                .@"trailing.field_offsets.len" = .@"payload.fields_len",
            },
        },
        .type_struct_packed_auto = struct_packed_encoding,
        .type_struct_packed_explicit = struct_packed_encoding,
        .type_struct_packed_auto_defaults = struct_packed_defaults_encoding,
        .type_struct_packed_explicit_defaults = struct_packed_defaults_encoding,
        .type_union = .{
            .summary = .@"{.payload.name%summary#\"}",
            .payload = TypeUnion,
            .trailing = struct {
                type_hash: ?u64,
                captures_len: ?u32,
                captures: ?[]CaptureValue,
                field_types: []Index,
                field_aligns: ?[]Alignment,
            },
            .config = .{
                .@"trailing.type_hash.?" = .@"payload.flags.any_captures == .reified",
                .@"trailing.captures_len.?" = .@"payload.flags.any_captures == .true",
                .@"trailing.captures.?" = .@"payload.flags.any_captures == .true",
                .@"trailing.captures.?.len" = .@"trailing.captures_len.?",
                .@"trailing.field_types.len" = .@"payload.fields_len",
                .@"trailing.field_aligns.?" = .@"payloads.flags.any_field_aligns",
                .@"trailing.field_aligns.?.len" = .@"payload.fields_len",
            },
        },
        .type_union_packed_auto = union_packed_encoding,
        .type_union_packed_explicit = union_packed_encoding,
        .type_enum_auto = .{
            .summary = .@"{.payload.name%summary#\"}",
            .payload = TypeEnum,
            .trailing = struct {
                owner_union: ?Index,
                zir_index: ?TrackedInst.Index,
                type_hash: ?u64,
                captures: ?[]CaptureValue,
                field_names: []NullTerminatedString,
            },
            .config = .{
                .@"trailing.owner_union.?" = .@"payload.captures_len == .generated_union_tag",
                .@"trailing.zir_index.?" = .@"payload.captures_len != .generated_union_tag",
                .@"trailing.type_hash.?" = .@"payload.captures_len == .reified",
                .@"trailing.captures.?" = .@"payload.captures_len != .reified and payload.captures_len != .generated_enum_tag",
                .@"trailing.captures.?.len" = .@"@intFromEnum(payload.captures_len)",
                .@"trailing.field_names.len" = .@"payload.fields_len",
            },
        },
        .type_enum_explicit = enum_explicit_encoding,
        .type_enum_nonexhaustive = enum_explicit_encoding,
        .type_opaque = .{
            .summary = .@"{.payload.name%summary#\"}",
            .payload = TypeOpaque,
            .trailing = struct { captures: []CaptureValue },
            .config = .{ .@"trailing.captures.len" = .@"payload.captures_len" },
        },

        .undef = .{ .summary = .@"@as({.data%summary}, undefined)", .data = Index },
        .simple_value = .{ .summary = .@"{.index%value#.}", .index = SimpleValue },
        .ptr_nav = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&{.payload.nav.fqn%summary#\"}) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrNav,
        },
        .ptr_comptime_alloc = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&comptime_allocs[{.payload.index%summary}]) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrComptimeAlloc,
        },
        .ptr_uav = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&{.payload.val%summary}) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrUav,
        },
        .ptr_uav_aligned = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(@as({.payload.orig_ty%summary}, &{.payload.val%summary})) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrUavAligned,
        },
        .ptr_comptime_field = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&{.payload.field_val%summary}) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrComptimeField,
        },
        .ptr_int = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value}))",
            .payload = PtrInt,
        },
        .ptr_eu_payload = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&({.payload.base%summary} catch unreachable)) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrBase,
        },
        .ptr_opt_payload = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&{.payload.base%summary}.?) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrBase,
        },
        .ptr_elem = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&{.payload.base%summary}[{.payload.index%summary}]) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrBaseIndex,
        },
        .ptr_field = .{
            .summary = .@"@as({.payload.ty%summary}, @ptrFromInt(@intFromPtr(&{.payload.base%summary}[{.payload.index%summary}]) + ({.payload.byte_offset_a%value} << 32 | {.payload.byte_offset_b%value})))",
            .payload = PtrBaseIndex,
        },
        .ptr_slice = .{
            .summary = .@"{.payload.ptr%summary}[0..{.payload.len%summary}]",
            .payload = PtrSlice,
        },
        .opt_payload = .{ .summary = .@"@as({.payload.ty%summary}, {.payload.val%summary})", .payload = TypeValue },
        .opt_null = .{ .summary = .@"@as({.data%summary}, null)", .data = Index },
        .int_u8 = .{ .summary = .@"@as(u8, {.data%value})", .data = u8 },
        .int_u16 = .{ .summary = .@"@as(u16, {.data%value})", .data = u16 },
        .int_u32 = .{ .summary = .@"@as(u32, {.data%value})", .data = u32 },
        .int_i32 = .{ .summary = .@"@as(i32, {.data%value})", .data = i32 },
        .int_usize = .{ .summary = .@"@as(usize, {.data%value})", .data = u32 },
        .int_comptime_int_u32 = .{ .summary = .@"{.data%value}", .data = u32 },
        .int_comptime_int_i32 = .{ .summary = .@"{.data%value}", .data = i32 },
        .int_small = .{ .summary = .@"@as({.payload.ty%summary}, {.payload.value%value})", .payload = IntSmall },
        .int_positive = .{},
        .int_negative = .{},
        .error_set_error = .{ .summary = .@"@as({.payload.ty%summary}, error.@{.payload.name%summary})", .payload = Error },
        .error_union_error = .{ .summary = .@"@as({.payload.ty%summary}, error.@{.payload.name%summary})", .payload = Error },
        .error_union_payload = .{ .summary = .@"@as({.payload.ty%summary}, {.payload.val%summary})", .payload = TypeValue },
        .enum_literal = .{ .summary = .@".@{.data%summary}", .data = NullTerminatedString },
        .enum_tag = .{ .summary = .@"@as({.payload.ty%summary}, @enumFromInt({.payload.int%summary}))", .payload = EnumTag },
        .float_f16 = .{ .summary = .@"@as(f16, {.data%value})", .data = f16 },
        .float_f32 = .{ .summary = .@"@as(f32, {.data%value})", .data = f32 },
        .float_f64 = .{ .summary = .@"@as(f64, {.payload%value})", .payload = f64 },
        .float_f80 = .{ .summary = .@"@as(f80, {.payload%value})", .payload = f80 },
        .float_f128 = .{ .summary = .@"@as(f128, {.payload%value})", .payload = f128 },
        .float_c_longdouble_f80 = .{ .summary = .@"@as(c_longdouble, {.payload%value})", .payload = f80 },
        .float_c_longdouble_f128 = .{ .summary = .@"@as(c_longdouble, {.payload%value})", .payload = f128 },
        .float_comptime_float = .{ .summary = .@"{.payload%value}", .payload = f128 },
        .variable = .{ .summary = .@"{.payload.owner_nav.fqn%summary#\"}", .payload = Variable },
        .threadlocal_variable = .{ .summary = .@"{.payload.owner_nav.fqn%summary#\"}", .payload = Variable },
        .@"extern" = .{ .summary = .@"{.payload.owner_nav.fqn%summary#\"}", .payload = Extern },
        .func_decl = .{
            .summary = .@"{.payload.owner_nav.fqn%summary#\"}",
            .payload = FuncDecl,
            .trailing = struct { inferred_error_set: ?Index },
            .config = .{ .@"trailing.inferred_error_set.?" = .@"payload.analysis.inferred_error_set" },
        },
        .func_instance = .{
            .summary = .@"{.payload.owner_nav.fqn%summary#\"}",
            .payload = FuncInstance,
            .trailing = struct {
                inferred_error_set: ?Index,
                param_values: []Index,
            },
            .config = .{
                .@"trailing.inferred_error_set.?" = .@"payload.analysis.inferred_error_set",
                .@"trailing.param_values.len" = .@"payload.ty.payload.params_len",
            },
        },
        .func_coerced = .{
            .summary = .@"@as(*const {.payload.ty%summary}, @ptrCast(&{.payload.func%summary})).*",
            .payload = FuncCoerced,
        },
        .only_possible_value = .{ .summary = .@"@as({.data%summary}, undefined)", .data = Index },
        .union_value = .{ .summary = .@"@as({.payload.ty%summary}, {})", .payload = Union },
        .bytes = .{ .summary = .@"@as({.payload.ty%summary}, {.payload.bytes%summary}.*)", .payload = Bytes },
        .aggregate = .{
            .summary = .@"@as({.payload.ty%summary}, .{...})",
            .payload = Aggregate,
            .trailing = struct { elements: []Index },
            .config = .{ .@"trailing.elements.len" = .@"payload.ty.payload.fields_len" },
        },
        .repeated = .{ .summary = .@"@as({.payload.ty%summary}, @splat({.payload.elem_val%summary}))", .payload = Repeated },
        .bitpack = .{ .summary = .@"@as({.payload.ty%summary}, {})", .payload = Key.Bitpack },

        .memoized_call = .{
            .summary = .@"@memoize({.payload.func%summary})",
            .payload = MemoizedCall,
            .trailing = struct { arg_values: []Index },
            .config = .{ .@"trailing.arg_values.len" = .@"payload.args_len" },
        },
    };
    fn Payload(comptime tag: Tag) type {
        return @field(encodings, @tagName(tag)).payload;
    }

    pub const Variable = struct {
        ty: Index,
        /// May be `none`.
        init: Index,
        owner_nav: Nav.Index,
    };

    pub const Extern = struct {
        // name, is_const, alignment, addrspace come from `owner_nav`.
        ty: Index,
        lib_name: OptionalNullTerminatedString,
        flags: Flags,
        owner_nav: Nav.Index,
        zir_index: TrackedInst.Index,
        location_or_descriptor_set: u32,
        descriptor_binding: u32,

        pub const Flags = packed struct(u32) {
            linkage: std.builtin.GlobalLinkage,
            visibility: std.builtin.SymbolVisibility,
            is_threadlocal: bool,
            is_dll_import: bool,
            relocation: std.builtin.ExternOptions.Relocation,
            source: Source,
            decoration_type: DecorationType,
            _: u22 = 0,

            pub const Source = enum(u1) { builtin, syntax };
            pub const DecorationType = enum(u2) { none, location, descriptor };
        };

        pub fn decoration(self: Extern) ?std.builtin.ExternOptions.Decoration {
            return switch (self.flags.decoration_type) {
                .none => null,
                .location => std.builtin.ExternOptions.Decoration{
                    .location = self.location_or_descriptor_set,
                },
                .descriptor => std.builtin.ExternOptions.Decoration{ .descriptor = .{ .set = self.location_or_descriptor_set, .binding = self.descriptor_binding } },
            };
        }
    };

    /// Trailing:
    /// 0. element: Index for each len
    /// len is determined by the aggregate type.
    pub const Aggregate = struct {
        /// The type of the aggregate.
        ty: Index,
    };

    /// Trailing:
    /// 0. If `analysis.inferred_error_set` is `true`, `Index` of an `error_set` which
    ///    is a regular error set corresponding to the finished inferred error set.
    ///    A `none` value marks that the inferred error set is not resolved yet.
    pub const FuncDecl = struct {
        analysis: FuncAnalysis,
        owner_nav: Nav.Index,
        ty: Index,
        zir_body_inst: TrackedInst.Index,
        lbrace_line: u32,
        rbrace_line: u32,
        lbrace_column: u32,
        rbrace_column: u32,
    };

    /// Trailing:
    /// 0. If `analysis.inferred_error_set` is `true`, `Index` of an `error_set` which
    ///    is a regular error set corresponding to the finished inferred error set.
    ///    A `none` value marks that the inferred error set is not resolved yet.
    /// 1. For each parameter of generic_owner: `Index` if comptime, otherwise `none`
    pub const FuncInstance = struct {
        analysis: FuncAnalysis,
        // Needed by the linker for codegen. Not part of hashing or equality.
        owner_nav: Nav.Index,
        ty: Index,
        branch_quota: u32,
        /// Points to a `FuncDecl`.
        generic_owner: Index,
    };

    pub const FuncCoerced = struct {
        ty: Index,
        func: Index,
    };

    /// Trailing:
    /// 0. name: NullTerminatedString for each names_len
    pub const ErrorSet = struct {
        names_len: u32,
        /// Maps error names to declaration index.
        names_map: MapIndex,
    };

    /// Trailing:
    /// 0. comptime_bits: u32, // if has_comptime_bits
    /// 1. noalias_bits: u32, // if has_noalias_bits
    /// 2. param_type: Index for each params_len
    pub const TypeFunction = struct {
        params_len: u32,
        return_type: Index,
        flags: Flags,

        pub const Flags = packed struct(u32) {
            cc: PackedCallingConvention,
            is_var_args: bool,
            has_comptime_bits: bool,
            has_noalias_bits: bool,
            is_noinline: bool,
            _: u10 = 0,
        };
    };

    /// At first I thought of storing the denormalized data externally, such as...
    ///
    /// * runtime field order
    /// * calculated field offsets
    /// * size and alignment of the struct
    ///
    /// ...since these can be computed based on the other data here. However,
    /// this data does need to be memoized, and therefore stored in memory
    /// while the compiler is running, in order to avoid O(N^2) logic in many
    /// places. Since the data can be stored compactly in the InternPool
    /// representation, it is better for memory usage to store denormalized data
    /// here, and potentially also better for performance as well. It's also simpler
    /// than coming up with some other scheme for the data.
    ///
    /// Trailing:
    /// 0. type_hash: PackedU64  // if `any_captures == .reified`
    /// 1. captures_len: u32     // if `any_captures == .true`
    /// 2. capture: CaptureValue // for each `captures_len`
    /// 3. field_name: NullTerminatedString // for each `fields_len`
    /// 4. field_type: Index           // for each `fields_len`
    /// 5. field_default: Index        // if `any_field_defaults`; for each `fields_len`
    /// 6. field_align: Alignment      // if `any_field_aligns`; for each `fields_len`
    /// 7. field_is_comptime_bits: u32 // if `any_comptime_fields`; minimum `u32` for `fields_len`; LSB is field 0
    /// 8. field_runtime_order: RuntimeOrder // if `layout == .auto`; for each `fields_len`
    /// 9. field_offset: u32                 // for each `fields_len`
    pub const TypeStruct = struct {
        zir_index: TrackedInst.Index,

        name: NullTerminatedString,
        name_nav: Nav.Index.Optional,
        namespace: NamespaceIndex,

        fields_len: u32,
        field_name_map: MapIndex,

        /// Size in bytes of the whole struct. Always 0 until layout resolved.
        size: u32,

        flags: Flags,

        pub const Flags = packed struct(u32) {
            any_captures: enum(u2) { true, false, reified },

            /// `packed` layout is represented separately by `TypeStructPacked`.
            layout: enum(u1) { auto, @"extern" },

            any_comptime_fields: bool,
            any_field_defaults: bool,
            any_field_aligns: bool,

            class: TypeClass,
            /// Alignment of the whole struct. Always `.none` until layout resolved.
            alignment: Alignment,

            want_layout: bool,

            _: u16 = 0,
        };
    };

    /// Trailing:
    /// 0. type_hash: PackedU64  // if `captures_len == .reified`
    /// 1. capture: CaptureValue // if `captures_len != .reified`; for each `captures_len`
    /// 2. field_name: NullTerminatedString // for each `fields_len`
    /// 3. field_type: Index                // for each `fields_len`
    /// 4. field_default: Index             // if item tag implies field defaults; for each `fields_len`
    pub const TypeStructPacked = struct {
        zir_index: TrackedInst.Index,
        bits: Bits,

        name: NullTerminatedString,
        name_nav: Nav.Index.Optional,
        namespace: NamespaceIndex,

        /// The corresponding `BackingTypeMode` depends on the item's `Tag`.
        backing_int_type: Index,

        fields_len: u32,
        field_name_map: MapIndex,

        const Bits = packed struct(u32) {
            captures_len: enum(u31) {
                reified = std.math.maxInt(u31),
                _,
            },
            want_layout: bool,
        };
    };

    /// For declared unions, field names are intentionally omitted because they are available in
    /// `enum_tag_type`. However, reified unions do store field names, because they are needed by
    /// type resolution to create or validate the enum tag type (type resolution for declared unions
    /// instead fetches field names from ZIR).
    ///
    /// Trailing:
    /// 0. type_hash: PackedU64   // if `any_captures == .reified`
    /// 1. captures_len: u32      // if `any_captures == .true`
    /// 2. capture: CaptureValue  // if `any_captures == .true`; for each `captures_len`
    /// 3. reified_field_name: NullTerminatedString // if `any_captures == .reified`; for each `fields_len`
    /// 4. field_type: Index      // for each `fields_len`
    /// 5. field_align: Alignment // for each `fields_len` if `any_field_aligns`
    pub const TypeUnion = struct {
        zir_index: TrackedInst.Index,

        name: NullTerminatedString,
        name_nav: Nav.Index.Optional,
        namespace: NamespaceIndex,
        /// The enum that provides the list of field names and values.
        enum_tag_type: Index,

        /// This could be provided through the tag type, but it is more convenient
        /// to store it directly. This is also necessary for `dumpStatsFallible` to
        /// work on unresolved types.
        fields_len: u32,

        /// Always 0 until layout resolved.
        size: u32,
        /// Always 0 until layout resolved.
        padding: u32,

        flags: Flags,

        pub const Flags = packed struct(u32) {
            any_captures: enum(u2) { true, false, reified },

            /// Whether `enum_tag_type` was explicitly specified with `union(E)` syntax.
            ///
            /// For `union(enum(E))` syntax, this is `false`, but the generated enum tag type is
            /// considered to have an explicitly specified integer tag type.
            enum_tag_mode: BackingTypeMode,

            /// `packed` layout is represented separately by `TypeStructPacked`.
            layout: enum(u1) { auto, @"extern" },

            any_field_aligns: bool,
            tag_usage: LoadedUnionType.TagUsage,

            class: TypeClass,
            has_runtime_tag: bool,

            /// Alignment of the whole union. Always `.none` until layout resolved.
            alignment: Alignment,

            want_layout: bool,

            _: u14 = 0,
        };
    };

    /// For declared unions, field names are intentionally omitted because they are available in
    /// `enum_tag_type`. However, reified unions do store field names, because they are needed by
    /// type resolution to create or validate the enum tag type (type resolution for declared unions
    /// instead fetches field names from ZIR).
    ///
    /// Trailing:
    /// 0. type_hash: PackedU64  // if `captures_len == .reified`
    /// 1. capture: CaptureValue // if `captures_len != .reified`; for each `captures_len`
    /// 2. reified_field_name: NullTerminatedString // if `captures_len == .reified`; for each `fields_len`
    /// 3. field_type: Index     // for each `fields_len`
    pub const TypeUnionPacked = struct {
        zir_index: TrackedInst.Index,
        bits: Bits,

        name: NullTerminatedString,
        name_nav: Nav.Index.Optional,
        namespace: NamespaceIndex,

        /// The corresponding `BackingTypeMode` depends on the item's `Tag`.
        backing_int_type: Index,
        /// Although packed unions do not semantically have a tag type, the compiler still assigns
        /// them a "hypothetical" tag type.
        enum_tag_type: Index,

        /// This could be provided through the tag type, but it is more convenient
        /// to store it directly. This is also necessary for `dumpStatsFallible` to
        /// work on unresolved types.
        fields_len: u32,

        const Bits = packed struct(u32) {
            captures_len: enum(u31) {
                reified = std.math.maxInt(u31),
                _,
            },
            want_layout: bool,
        };
    };

    /// Trailing:
    /// 0. owner_union: Index               // if `captures_len == .generated_union_tag`
    /// 1. zir_index: TrackedInst.Index     // if `captures_len != .generated_union_tag`
    /// 2. type_hash: PackedU64             // if `captures_len == .reified`
    /// 3. capture: CaptureValue            // if `captures_len` is not a named tag; for each `captures_len`
    /// 4. field_value_map: MapIndex        // if tag is not `.type_enum_auto`
    /// 5. field_name: NullTerminatedString // for each `fields_len`
    /// 6. field_value: Index               // if tag is not `.type_enum_auto`; for each `fields_len`
    pub const TypeEnum = struct {
        bits: Bits,

        name: NullTerminatedString,
        name_nav: Nav.Index.Optional,
        namespace: NamespaceIndex,

        /// An integer type which is used for the numerical value of the enum. Whether this was
        /// user-provided or inferred by the compiler depends on the tag.
        int_tag_type: Index,

        fields_len: u32,
        field_name_map: MapIndex,

        const Bits = packed struct(u32) {
            captures_len: enum(u31) {
                reified = std.math.maxInt(u31),
                generated_union_tag = std.math.maxInt(u31) - 1,
                _,
            },
            want_layout: bool,
        };
    };

    /// Trailing:
    /// 0. capture: CaptureValue // for each `captures_len`
    pub const TypeOpaque = struct {
        zir_index: TrackedInst.Index,
        captures_len: u32,

        name: NullTerminatedString,
        name_nav: Nav.Index.Optional,
        namespace: NamespaceIndex,
    };
};

/// Differentiates between user-provided and compiler-generated backing types for packed and tagged types.
pub const BackingTypeMode = enum(u1) {
    /// The backing type was explicitly provided by the user. For instance:
    ///   union(T)
    ///   enum(T)
    ///   packed struct(T)
    ///   packed union(T)
    /// Type layout resolution will evaluate the user-provided expression and validate that type.
    explicit,
    /// No backing type was explicitly provided by the user. Type layout resolution will populate
    /// an inferred/generated type.
    auto,
};

/// State that is mutable during semantic analysis. This data is not used for
/// equality or hashing, except for `inferred_error_set` which is considered
/// to be part of the type of the function.
pub const FuncAnalysis = packed struct(u32) {
    want_runtime_analysis: bool,
    branch_hint: std.builtin.BranchHint,
    is_noinline: bool,
    has_error_trace: bool,
    /// True if this function has an inferred error set.
    inferred_error_set: bool,
    disable_instrumentation: bool,
    disable_intrinsics: bool,

    _: u23 = 0,
};

pub const Bytes = struct {
    /// The type of the aggregate
    ty: Index,
    /// Index into strings, of len ip.aggregateTypeLen(ty)
    bytes: String,
};

pub const Repeated = struct {
    /// The type of the aggregate.
    ty: Index,
    /// The value of every element.
    elem_val: Index,
};

/// Trailing:
/// 0. type: Index for each fields_len
/// 1. value: Index for each fields_len
pub const TypeTuple = struct {
    fields_len: u32,
};

/// Having `SimpleType` and `SimpleValue` in separate enums makes it easier to
/// implement logic that only wants to deal with types because the logic can
/// ignore all simple values. Note that technically, types are values.
pub const SimpleType = enum(u32) {
    f16 = @intFromEnum(Index.f16_type),
    f32 = @intFromEnum(Index.f32_type),
    f64 = @intFromEnum(Index.f64_type),
    f80 = @intFromEnum(Index.f80_type),
    f128 = @intFromEnum(Index.f128_type),
    usize = @intFromEnum(Index.usize_type),
    isize = @intFromEnum(Index.isize_type),
    c_char = @intFromEnum(Index.c_char_type),
    c_short = @intFromEnum(Index.c_short_type),
    c_ushort = @intFromEnum(Index.c_ushort_type),
    c_int = @intFromEnum(Index.c_int_type),
    c_uint = @intFromEnum(Index.c_uint_type),
    c_long = @intFromEnum(Index.c_long_type),
    c_ulong = @intFromEnum(Index.c_ulong_type),
    c_longlong = @intFromEnum(Index.c_longlong_type),
    c_ulonglong = @intFromEnum(Index.c_ulonglong_type),
    c_longdouble = @intFromEnum(Index.c_longdouble_type),
    anyopaque = @intFromEnum(Index.anyopaque_type),
    bool = @intFromEnum(Index.bool_type),
    void = @intFromEnum(Index.void_type),
    type = @intFromEnum(Index.type_type),
    anyerror = @intFromEnum(Index.anyerror_type),
    comptime_int = @intFromEnum(Index.comptime_int_type),
    comptime_float = @intFromEnum(Index.comptime_float_type),
    noreturn = @intFromEnum(Index.noreturn_type),
    null = @intFromEnum(Index.null_type),
    undefined = @intFromEnum(Index.undefined_type),
    enum_literal = @intFromEnum(Index.enum_literal_type),

    adhoc_inferred_error_set = @intFromEnum(Index.adhoc_inferred_error_set_type),
    generic_poison = @intFromEnum(Index.generic_poison_type),
};

pub const SimpleValue = enum(u32) {
    void = @intFromEnum(Index.void_value),
    /// This is untyped `null`.
    null = @intFromEnum(Index.null_value),
    true = @intFromEnum(Index.bool_true),
    false = @intFromEnum(Index.bool_false),
    @"unreachable" = @intFromEnum(Index.unreachable_value),
};

/// Stored as a power-of-two, with one special value to indicate none.
pub const Alignment = enum(u6) {
    @"1" = 0,
    @"2" = 1,
    @"4" = 2,
    @"8" = 3,
    @"16" = 4,
    @"32" = 5,
    @"64" = 6,
    none = std.math.maxInt(u6),
    _,

    pub fn toByteUnits(a: Alignment) ?u64 {
        return switch (a) {
            .none => null,
            else => @as(u64, 1) << @intFromEnum(a),
        };
    }

    pub fn fromByteUnits(n: u64) Alignment {
        if (n == 0) return .none;
        assert(std.math.isPowerOfTwo(n));
        return @enumFromInt(@ctz(n));
    }

    pub fn fromNonzeroByteUnits(n: u64) Alignment {
        assert(n != 0);
        return fromByteUnits(n);
    }

    pub fn toLog2Units(a: Alignment) u6 {
        assert(a != .none);
        return @intFromEnum(a);
    }

    /// This is just a glorified `@enumFromInt` but using it can help
    /// document the intended conversion.
    /// The parameter uses a u32 for convenience at the callsite.
    pub fn fromLog2Units(a: u32) Alignment {
        assert(a != @intFromEnum(Alignment.none));
        return @enumFromInt(a);
    }

    pub fn order(lhs: Alignment, rhs: Alignment) std.math.Order {
        assert(lhs != .none);
        assert(rhs != .none);
        return std.math.order(@intFromEnum(lhs), @intFromEnum(rhs));
    }

    /// Relaxed comparison. We have this as default because a lot of callsites
    /// were upgraded from directly using comparison operators on byte units,
    /// with the `none` value represented by zero.
    /// Prefer `compareStrict` if possible.
    pub fn compare(lhs: Alignment, op: std.math.CompareOperator, rhs: Alignment) bool {
        return std.math.compare(lhs.toRelaxedCompareUnits(), op, rhs.toRelaxedCompareUnits());
    }

    pub fn compareStrict(lhs: Alignment, op: std.math.CompareOperator, rhs: Alignment) bool {
        assert(lhs != .none);
        assert(rhs != .none);
        return std.math.compare(@intFromEnum(lhs), op, @intFromEnum(rhs));
    }

    /// Treats `none` as zero.
    /// This matches previous behavior of using `@max` directly on byte units.
    /// Prefer `maxStrict` if possible.
    pub fn max(lhs: Alignment, rhs: Alignment) Alignment {
        if (lhs == .none) return rhs;
        if (rhs == .none) return lhs;
        return maxStrict(lhs, rhs);
    }

    pub fn maxStrict(lhs: Alignment, rhs: Alignment) Alignment {
        assert(lhs != .none);
        assert(rhs != .none);
        return @enumFromInt(@max(@intFromEnum(lhs), @intFromEnum(rhs)));
    }

    /// Treats `none` as zero.
    /// This matches previous behavior of using `@min` directly on byte units.
    /// Prefer `minStrict` if possible.
    pub fn min(lhs: Alignment, rhs: Alignment) Alignment {
        if (lhs == .none) return lhs;
        if (rhs == .none) return rhs;
        return minStrict(lhs, rhs);
    }

    pub fn minStrict(lhs: Alignment, rhs: Alignment) Alignment {
        assert(lhs != .none);
        assert(rhs != .none);
        return @enumFromInt(@min(@intFromEnum(lhs), @intFromEnum(rhs)));
    }

    /// Align an address forwards to this alignment.
    pub fn forward(a: Alignment, addr: u64) u64 {
        assert(a != .none);
        const x = (@as(u64, 1) << @intFromEnum(a)) - 1;
        return (addr + x) & ~x;
    }

    /// Align an address backwards to this alignment.
    pub fn backward(a: Alignment, addr: u64) u64 {
        assert(a != .none);
        const x = (@as(u64, 1) << @intFromEnum(a)) - 1;
        return addr & ~x;
    }

    /// Check if an address is aligned to this amount.
    pub fn check(a: Alignment, addr: u64) bool {
        assert(a != .none);
        return @ctz(addr) >= @intFromEnum(a);
    }

    /// An array of `Alignment` objects existing within the `extra` array.
    /// This type exists to provide a struct with lifetime that is
    /// not invalidated when items are added to the `InternPool`.
    pub const Slice = struct {
        tid: Zcu.PerThread.Id,
        start: u32,
        /// This is the number of alignment values, not the number of u32 elements.
        len: u32,

        pub const empty: Slice = .{ .tid = .main, .start = 0, .len = 0 };

        pub fn get(slice: Slice, ip: *const InternPool) []Alignment {
            const extra = ip.getLocalShared(slice.tid).extra.acquire();
            const bytes: []u8 = @ptrCast(extra.view().items(.@"0")[slice.start..]);
            return @ptrCast(bytes[0..slice.len]);
        }

        /// If `slice` is empty (`slice.len == 0`), returns `.none`.
        /// Otherwise, asserts that `index < slice.len`, and returns the value at `index`.
        pub fn getOrNone(slice: Slice, ip: *const InternPool, index: usize) Alignment {
            if (slice.len == 0) return .none;
            return slice.get(ip)[index];
        }
    };

    pub fn toRelaxedCompareUnits(a: Alignment) u8 {
        const n: u8 = @intFromEnum(a);
        assert(n <= @intFromEnum(Alignment.none));
        if (n == @intFromEnum(Alignment.none)) return 0;
        return n + 1;
    }

    pub fn toStdMem(a: Alignment) std.mem.Alignment {
        assert(a != .none);
        return @enumFromInt(@intFromEnum(a));
    }

    pub fn fromStdMem(a: std.mem.Alignment) Alignment {
        const r: Alignment = @enumFromInt(@intFromEnum(a));
        assert(r != .none);
        return r;
    }

    const LlvmBuilderAlignment = std.zig.llvm.Builder.Alignment;

    pub fn toLlvm(a: Alignment) LlvmBuilderAlignment {
        return @enumFromInt(@intFromEnum(a));
    }

    pub fn fromLlvm(a: LlvmBuilderAlignment) Alignment {
        return @enumFromInt(@intFromEnum(a));
    }
};

/// Used for non-sentineled arrays that have length fitting in u32, as well as
/// vectors.
pub const Vector = struct {
    len: u32,
    child: Index,
};

pub const Array = struct {
    len0: u32,
    len1: u32,
    child: Index,
    sentinel: Index,

    pub const Length = PackedU64;

    pub fn getLength(a: Array) u64 {
        return (PackedU64{
            .a = a.len0,
            .b = a.len1,
        }).get();
    }
};

pub const PackedU64 = packed struct(u64) {
    a: u32,
    b: u32,

    pub fn get(x: PackedU64) u64 {
        return @bitCast(x);
    }

    pub fn init(x: u64) PackedU64 {
        return @bitCast(x);
    }
};

pub const PtrNav = struct {
    ty: Index,
    nav: Nav.Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, nav: Nav.Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .nav = nav,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrUav = struct {
    ty: Index,
    val: Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, val: Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .val = val,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrUavAligned = struct {
    ty: Index,
    val: Index,
    /// Must be nonequal to `ty`. Only the alignment from this value is important.
    orig_ty: Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, val: Index, orig_ty: Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .val = val,
            .orig_ty = orig_ty,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrComptimeAlloc = struct {
    ty: Index,
    index: ComptimeAllocIndex,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, index: ComptimeAllocIndex, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .index = index,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrComptimeField = struct {
    ty: Index,
    field_val: Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, field_val: Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .field_val = field_val,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrBase = struct {
    ty: Index,
    base: Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, base: Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .base = base,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrBaseIndex = struct {
    ty: Index,
    base: Index,
    index: Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, base: Index, index: Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .base = base,
            .index = index,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrInt = struct {
    ty: Index,
    byte_offset_a: u32,
    byte_offset_b: u32,
    fn init(ty: Index, byte_offset: u64) @This() {
        return .{
            .ty = ty,
            .byte_offset_a = @intCast(byte_offset >> 32),
            .byte_offset_b = @truncate(byte_offset),
        };
    }
    fn byteOffset(data: @This()) u64 {
        return @as(u64, data.byte_offset_a) << 32 | data.byte_offset_b;
    }
};

pub const PtrSlice = struct {
    /// The slice type.
    ty: Index,
    /// A many pointer value.
    ptr: Index,
    /// A usize value.
    len: Index,
};

/// Trailing: Limb for every limbs_len
pub const Int = packed struct {
    ty: Index,
    limbs_len: u32,

    const limbs_items_len = @divExact(@sizeOf(Int), @sizeOf(Limb));
};

pub const IntSmall = struct {
    ty: Index,
    value: u32,
};

/// A f64 value, broken up into 2 u32 parts.
pub const Float64 = struct {
    piece0: u32,
    piece1: u32,

    pub fn get(self: Float64) f64 {
        const int_bits = @as(u64, self.piece0) | (@as(u64, self.piece1) << 32);
        return @bitCast(int_bits);
    }

    fn pack(val: f64) Float64 {
        const bits: u64 = @bitCast(val);
        return .{
            .piece0 = @truncate(bits),
            .piece1 = @truncate(bits >> 32),
        };
    }
};

/// A f80 value, broken up into 2 u32 parts and a u16 part zero-padded to a u32.
pub const Float80 = struct {
    piece0: u32,
    piece1: u32,
    piece2: u32, // u16 part, top bits

    pub fn get(self: Float80) f80 {
        const int_bits = @as(u80, self.piece0) |
            (@as(u80, self.piece1) << 32) |
            (@as(u80, self.piece2) << 64);
        return @bitCast(int_bits);
    }

    fn pack(val: f80) Float80 {
        const bits: u80 = @bitCast(val);
        return .{
            .piece0 = @truncate(bits),
            .piece1 = @truncate(bits >> 32),
            .piece2 = @truncate(bits >> 64),
        };
    }
};

/// A f128 value, broken up into 4 u32 parts.
pub const Float128 = struct {
    piece0: u32,
    piece1: u32,
    piece2: u32,
    piece3: u32,

    pub fn get(self: Float128) f128 {
        const int_bits = @as(u128, self.piece0) |
            (@as(u128, self.piece1) << 32) |
            (@as(u128, self.piece2) << 64) |
            (@as(u128, self.piece3) << 96);
        return @bitCast(int_bits);
    }

    fn pack(val: f128) Float128 {
        const bits: u128 = @bitCast(val);
        return .{
            .piece0 = @truncate(bits),
            .piece1 = @truncate(bits >> 32),
            .piece2 = @truncate(bits >> 64),
            .piece3 = @truncate(bits >> 96),
        };
    }
};

/// Trailing:
/// 0. arg value: Index for each args_len
pub const MemoizedCall = struct {
    func: Index,
    args_len: u32,
    result: Index,
    branch_count: u32,
};

pub fn init(ip: *InternPool, gpa: Allocator, io: Io, available_threads: usize) !void {
    errdefer ip.deinit(gpa, io);
    assert(ip.locals.len == 0 and ip.shards.len == 0);
    assert(available_threads > 0 and available_threads <= std.math.maxInt(u8));

    const used_threads = if (single_threaded) 1 else @max(available_threads, 2);
    ip.locals = try gpa.alloc(Local, used_threads);
    @memset(ip.locals, .{
        .shared = .{
            .items = .empty,
            .extra = .empty,
            .limbs = .empty,
            .strings = .empty,
            .string_bytes = .empty,
            .tracked_insts = .empty,
            .files = .empty,
            .maps = .empty,
            .navs = .empty,
            .comptime_units = .empty,

            .namespaces = .empty,
        },
        .mutate = .{
            .arena = .{},

            .items = .empty,
            .extra = .empty,
            .limbs = .empty,
            .strings = .empty,
            .string_bytes = .empty,
            .tracked_insts = .empty,
            .files = .empty,
            .maps = .empty,
            .navs = .empty,
            .comptime_units = .empty,

            .namespaces = .empty,
        },
    });
    for (ip.locals) |*local| try local.getMutableStrings(gpa, io).append(.{0});

    ip.tid_width = @intCast(std.math.log2_int_ceil(usize, used_threads));
    ip.tid_shift_30 = if (single_threaded) 0 else 30 - ip.tid_width;
    ip.tid_shift_31 = if (single_threaded) 0 else 31 - ip.tid_width;
    ip.tid_shift_32 = if (single_threaded) 0 else ip.tid_shift_31 +| 1;
    ip.shards = try gpa.alloc(Shard, @as(usize, 1) << ip.tid_width);
    @memset(ip.shards, .{
        .shared = .{
            .map = .empty,
            .string_map = .empty,
            .tracked_inst_map = .empty,
        },
        .mutate = .{
            .map = .empty,
            .string_map = .empty,
            .tracked_inst_map = .empty,
        },
    });

    // Reserve string index 0 for an empty string.
    assert((try ip.getOrPutString(gpa, io, .main, "", .no_embedded_nulls)) == .empty);

    // This inserts all the statically-known values into the intern pool in the
    // order expected.
    for (&static_keys, 0..) |key, key_index| switch (@as(Index, @enumFromInt(key_index))) {
        .empty_tuple_type => assert(try ip.getTupleType(gpa, io, .main, .{
            .types = &.{},
            .values = &.{},
        }) == .empty_tuple_type),
        else => |expected_index| assert(try ip.get(gpa, io, .main, key) == expected_index),
    };

    if (std.debug.runtime_safety) {
        // Sanity check.
        assert(ip.indexToKey(.bool_true).simple_value == .true);
        assert(ip.indexToKey(.bool_false).simple_value == .false);
    }
}

pub fn deinit(ip: *InternPool, gpa: Allocator, io: Io) void {
    if (debug_state.enable_checks) std.debug.assert(debug_state.intern_pool == null);

    ip.src_hash_deps.deinit(gpa);
    ip.nav_val_deps.deinit(gpa);
    ip.nav_ty_deps.deinit(gpa);
    ip.func_ies_deps.deinit(gpa);
    ip.type_layout_deps.deinit(gpa);
    ip.zon_file_deps.deinit(gpa);
    ip.embed_file_deps.deinit(gpa);
    ip.namespace_deps.deinit(gpa);
    ip.namespace_name_deps.deinit(gpa);

    ip.first_dependency.deinit(gpa);

    ip.dep_entries.deinit(gpa);
    ip.free_dep_entries.deinit(gpa);

    gpa.free(ip.shards);
    for (ip.locals) |*local| {
        const buckets_len = local.mutate.namespaces.buckets_list.len;
        if (buckets_len > 0) for (
            local.shared.namespaces.view().items(.@"0")[0..buckets_len],
            0..,
        ) |namespace_bucket, buckets_index| {
            for (namespace_bucket[0..if (buckets_index < buckets_len - 1)
                namespace_bucket.len
            else
                local.mutate.namespaces.last_bucket_len]) |*namespace|
            {
                namespace.pub_decls.deinit(gpa);
                namespace.priv_decls.deinit(gpa);
                namespace.comptime_decls.deinit(gpa);
                namespace.test_decls.deinit(gpa);
            }
        };
        const maps = local.getMutableMaps(gpa, io);
        if (maps.mutate.len > 0) for (maps.view().items(.@"0")) |*map| map.deinit(gpa);
        local.mutate.arena.promote(gpa).deinit();
    }
    gpa.free(ip.locals);

    ip.* = undefined;
}

pub fn activate(ip: *const InternPool) void {
    if (!debug_state.enable) return;
    _ = Index.Unwrapped.debug_state;
    _ = String.debug_state;
    _ = OptionalString.debug_state;
    _ = NullTerminatedString.debug_state;
    _ = OptionalNullTerminatedString.debug_state;
    _ = TrackedInst.Index.debug_state;
    _ = TrackedInst.Index.Optional.debug_state;
    _ = Nav.Index.debug_state;
    _ = Nav.Index.Optional.debug_state;
    if (debug_state.enable_checks) std.debug.assert(debug_state.intern_pool == null);
    debug_state.intern_pool = ip;
}

pub fn deactivate(ip: *const InternPool) void {
    if (!debug_state.enable) return;
    std.debug.assert(debug_state.intern_pool == ip);
    if (debug_state.enable_checks) debug_state.intern_pool = null;
}

/// For debugger access only.
const debug_state = struct {
    const enable = false;
    const enable_checks = enable and !builtin.single_threaded;
    threadlocal var intern_pool: ?*const InternPool = null;
};

pub fn indexToKey(ip: *const InternPool, index: Index) Key {
    assert(index != .none);
    const unwrapped_index = index.unwrap(ip);
    const item = unwrapped_index.getItem(ip);
    const data = item.data;
    return switch (item.tag) {
        .removed => unreachable,
        .type_int_signed => .{
            .int_type = .{
                .signedness = .signed,
                .bits = @intCast(data),
            },
        },
        .type_int_unsigned => .{
            .int_type = .{
                .signedness = .unsigned,
                .bits = @intCast(data),
            },
        },
        .type_array_big => {
            const array_info = extraData(unwrapped_index.getExtra(ip), Array, data);
            return .{ .array_type = .{
                .len = array_info.getLength(),
                .child = array_info.child,
                .sentinel = array_info.sentinel,
            } };
        },
        .type_array_small => {
            const array_info = extraData(unwrapped_index.getExtra(ip), Vector, data);
            return .{ .array_type = .{
                .len = array_info.len,
                .child = array_info.child,
                .sentinel = .none,
            } };
        },
        .simple_type => .{ .simple_type = @enumFromInt(@intFromEnum(index)) },
        .simple_value => .{ .simple_value = @enumFromInt(@intFromEnum(index)) },

        .type_vector => {
            const vector_info = extraData(unwrapped_index.getExtra(ip), Vector, data);
            return .{ .vector_type = .{
                .len = vector_info.len,
                .child = vector_info.child,
            } };
        },

        .type_pointer => .{ .ptr_type = extraData(unwrapped_index.getExtra(ip), Tag.TypePointer, data) },

        .type_slice => {
            const many_ptr_index: Index = @enumFromInt(data);
            const many_ptr_unwrapped = many_ptr_index.unwrap(ip);
            const many_ptr_item = many_ptr_unwrapped.getItem(ip);
            assert(many_ptr_item.tag == .type_pointer);
            var ptr_info = extraData(many_ptr_unwrapped.getExtra(ip), Tag.TypePointer, many_ptr_item.data);
            ptr_info.flags.size = .slice;
            return .{ .ptr_type = ptr_info };
        },

        .type_optional => .{ .opt_type = @enumFromInt(data) },
        .type_anyframe => .{ .anyframe_type = @enumFromInt(data) },

        .type_error_union => .{ .error_union_type = extraData(unwrapped_index.getExtra(ip), Key.ErrorUnionType, data) },
        .type_anyerror_union => .{ .error_union_type = .{
            .error_set_type = .anyerror_type,
            .payload_type = @enumFromInt(data),
        } },
        .type_error_set => .{ .error_set_type = extraErrorSet(unwrapped_index.tid, unwrapped_index.getExtra(ip), data) },
        .type_inferred_error_set => .{
            .inferred_error_set_type = @enumFromInt(data),
        },
        .type_function => .{ .func_type = extraFuncType(unwrapped_index.tid, unwrapped_index.getExtra(ip), data) },
        .type_tuple => .{ .tuple_type = extraTypeTuple(unwrapped_index.tid, unwrapped_index.getExtra(ip), data) },

        .type_struct => .{ .struct_type = ns: {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, Tag.TypeStruct, data);
            break :ns switch (extra.data.flags.any_captures) {
                .reified => .{ .reified = .{
                    .zir_index = extra.data.zir_index,
                    .type_hash = extraData(extra_list, PackedU64, extra.end).get(),
                } },
                .false => .{ .declared = .{
                    .zir_index = extra.data.zir_index,
                    .captures = .{ .owned = .empty },
                } },
                .true => .{ .declared = .{
                    .zir_index = extra.data.zir_index,
                    .captures = .{ .owned = .{
                        .tid = unwrapped_index.tid,
                        .start = extra.end + 1,
                        .len = extra_list.view().items(.@"0")[extra.end],
                    } },
                } },
            };
        } },
        .type_struct_packed_auto,
        .type_struct_packed_explicit,
        .type_struct_packed_auto_defaults,
        .type_struct_packed_explicit_defaults,
        => .{ .struct_type = ns: {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, Tag.TypeStructPacked, data);
            break :ns switch (extra.data.bits.captures_len) {
                .reified => .{ .reified = .{
                    .zir_index = extra.data.zir_index,
                    .type_hash = extraData(extra_list, PackedU64, extra.end).get(),
                } },
                _ => |len| .{ .declared = .{
                    .zir_index = extra.data.zir_index,
                    .captures = .{ .owned = .{
                        .tid = unwrapped_index.tid,
                        .start = extra.end,
                        .len = @intFromEnum(len),
                    } },
                } },
            };
        } },
        .type_union => .{ .union_type = ns: {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, Tag.TypeUnion, data);
            break :ns switch (extra.data.flags.any_captures) {
                .reified => .{ .reified = .{
                    .zir_index = extra.data.zir_index,
                    .type_hash = extraData(extra_list, PackedU64, extra.end).get(),
                } },
                .false => .{ .declared = .{
                    .zir_index = extra.data.zir_index,
                    .captures = .{ .owned = .empty },
                } },
                .true => .{ .declared = .{
                    .zir_index = extra.data.zir_index,
                    .captures = .{ .owned = .{
                        .tid = unwrapped_index.tid,
                        .start = extra.end + 1,
                        .len = extra_list.view().items(.@"0")[extra.end],
                    } },
                } },
            };
        } },
        .type_union_packed_auto, .type_union_packed_explicit => .{ .union_type = ns: {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, Tag.TypeUnionPacked, data);
            break :ns switch (extra.data.bits.captures_len) {
                .reified => .{ .reified = .{
                    .zir_index = extra.data.zir_index,
                    .type_hash = extraData(extra_list, PackedU64, extra.end).get(),
                } },
                _ => |len| .{ .declared = .{
                    .zir_index = extra.data.zir_index,
                    .captures = .{ .owned = .{
                        .tid = unwrapped_index.tid,
                        .start = extra.end,
                        .len = @intFromEnum(len),
                    } },
                } },
            };
        } },
        .type_enum_auto, .type_enum_explicit, .type_enum_nonexhaustive => .{ .enum_type = ns: {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, Tag.TypeEnum, data);
            break :ns switch (extra.data.bits.captures_len) {
                .reified => .{ .reified = .{
                    .zir_index = @enumFromInt(extra_list.view().items(.@"0")[extra.end]),
                    .type_hash = extraData(extra_list, PackedU64, extra.end + 1).get(),
                } },
                .generated_union_tag => .{ .generated_union_tag = owner_union: {
                    break :owner_union @enumFromInt(extra_list.view().items(.@"0")[extra.end]);
                } },
                _ => |len| .{ .declared = .{
                    .zir_index = @enumFromInt(extra_list.view().items(.@"0")[extra.end]),
                    .captures = .{ .owned = .{
                        .tid = unwrapped_index.tid,
                        .start = extra.end + 1,
                        .len = @intFromEnum(len),
                    } },
                } },
            };
        } },
        .type_opaque => .{ .opaque_type = ns: {
            const extra = extraDataTrail(unwrapped_index.getExtra(ip), Tag.TypeOpaque, data);
            break :ns .{ .declared = .{
                .zir_index = extra.data.zir_index,
                .captures = .{ .owned = .{
                    .tid = unwrapped_index.tid,
                    .start = extra.end,
                    .len = extra.data.captures_len,
                } },
            } };
        } },

        .undef => .{ .undef = @enumFromInt(data) },
        .opt_null => .{ .opt = .{
            .ty = @enumFromInt(data),
            .val = .none,
        } },
        .opt_payload => {
            const extra = extraData(unwrapped_index.getExtra(ip), Tag.TypeValue, data);
            return .{ .opt = .{
                .ty = extra.ty,
                .val = extra.val,
            } };
        },
        .ptr_nav => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrNav, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .nav = info.nav }, .byte_offset = info.byteOffset() } };
        },
        .ptr_comptime_alloc => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrComptimeAlloc, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .comptime_alloc = info.index }, .byte_offset = info.byteOffset() } };
        },
        .ptr_uav => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrUav, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .uav = .{
                .val = info.val,
                .orig_ty = info.ty,
            } }, .byte_offset = info.byteOffset() } };
        },
        .ptr_uav_aligned => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrUavAligned, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .uav = .{
                .val = info.val,
                .orig_ty = info.orig_ty,
            } }, .byte_offset = info.byteOffset() } };
        },
        .ptr_comptime_field => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrComptimeField, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .comptime_field = info.field_val }, .byte_offset = info.byteOffset() } };
        },
        .ptr_int => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrInt, data);
            return .{ .ptr = .{
                .ty = info.ty,
                .base_addr = .int,
                .byte_offset = info.byteOffset(),
            } };
        },
        .ptr_eu_payload => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrBase, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .eu_payload = info.base }, .byte_offset = info.byteOffset() } };
        },
        .ptr_opt_payload => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrBase, data);
            return .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .opt_payload = info.base }, .byte_offset = info.byteOffset() } };
        },
        .ptr_elem => {
            // Avoid `indexToKey` recursion by asserting the tag encoding.
            const info = extraData(unwrapped_index.getExtra(ip), PtrBaseIndex, data);
            const index_item = info.index.unwrap(ip).getItem(ip);
            return switch (index_item.tag) {
                .int_usize => .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .arr_elem = .{
                    .base = info.base,
                    .index = index_item.data,
                } }, .byte_offset = info.byteOffset() } },
                .int_positive => @panic("TODO"), // implement along with behavior test coverage
                else => unreachable,
            };
        },
        .ptr_field => {
            // Avoid `indexToKey` recursion by asserting the tag encoding.
            const info = extraData(unwrapped_index.getExtra(ip), PtrBaseIndex, data);
            const index_item = info.index.unwrap(ip).getItem(ip);
            return switch (index_item.tag) {
                .int_usize => .{ .ptr = .{ .ty = info.ty, .base_addr = .{ .field = .{
                    .base = info.base,
                    .index = index_item.data,
                } }, .byte_offset = info.byteOffset() } },
                .int_positive => @panic("TODO"), // implement along with behavior test coverage
                else => unreachable,
            };
        },
        .ptr_slice => {
            const info = extraData(unwrapped_index.getExtra(ip), PtrSlice, data);
            return .{ .slice = .{
                .ty = info.ty,
                .ptr = info.ptr,
                .len = info.len,
            } };
        },
        .int_u8 => .{ .int = .{
            .ty = .u8_type,
            .storage = .{ .u64 = data },
        } },
        .int_u16 => .{ .int = .{
            .ty = .u16_type,
            .storage = .{ .u64 = data },
        } },
        .int_u32 => .{ .int = .{
            .ty = .u32_type,
            .storage = .{ .u64 = data },
        } },
        .int_i32 => .{ .int = .{
            .ty = .i32_type,
            .storage = .{ .i64 = @as(i32, @bitCast(data)) },
        } },
        .int_usize => .{ .int = .{
            .ty = .usize_type,
            .storage = .{ .u64 = data },
        } },
        .int_comptime_int_u32 => .{ .int = .{
            .ty = .comptime_int_type,
            .storage = .{ .u64 = data },
        } },
        .int_comptime_int_i32 => .{ .int = .{
            .ty = .comptime_int_type,
            .storage = .{ .i64 = @as(i32, @bitCast(data)) },
        } },
        .int_positive => ip.indexToKeyBigInt(unwrapped_index.tid, data, true),
        .int_negative => ip.indexToKeyBigInt(unwrapped_index.tid, data, false),
        .int_small => {
            const info = extraData(unwrapped_index.getExtra(ip), IntSmall, data);
            return .{ .int = .{
                .ty = info.ty,
                .storage = .{ .u64 = info.value },
            } };
        },
        .float_f16 => .{ .float = .{
            .ty = .f16_type,
            .storage = .{ .f16 = @bitCast(@as(u16, @intCast(data))) },
        } },
        .float_f32 => .{ .float = .{
            .ty = .f32_type,
            .storage = .{ .f32 = @bitCast(data) },
        } },
        .float_f64 => .{ .float = .{
            .ty = .f64_type,
            .storage = .{ .f64 = extraData(unwrapped_index.getExtra(ip), Float64, data).get() },
        } },
        .float_f80 => .{ .float = .{
            .ty = .f80_type,
            .storage = .{ .f80 = extraData(unwrapped_index.getExtra(ip), Float80, data).get() },
        } },
        .float_f128 => .{ .float = .{
            .ty = .f128_type,
            .storage = .{ .f128 = extraData(unwrapped_index.getExtra(ip), Float128, data).get() },
        } },
        .float_c_longdouble_f80 => .{ .float = .{
            .ty = .c_longdouble_type,
            .storage = .{ .f80 = extraData(unwrapped_index.getExtra(ip), Float80, data).get() },
        } },
        .float_c_longdouble_f128 => .{ .float = .{
            .ty = .c_longdouble_type,
            .storage = .{ .f128 = extraData(unwrapped_index.getExtra(ip), Float128, data).get() },
        } },
        .float_comptime_float => .{ .float = .{
            .ty = .comptime_float_type,
            .storage = .{ .f128 = extraData(unwrapped_index.getExtra(ip), Float128, data).get() },
        } },
        .variable, .threadlocal_variable => {
            const extra = extraData(unwrapped_index.getExtra(ip), Tag.Variable, data);
            return .{ .variable = .{
                .ty = extra.ty,
                .init = extra.init,
                .owner_nav = extra.owner_nav,
                .is_threadlocal = switch (item.tag) {
                    else => unreachable,
                    .variable => false,
                    .threadlocal_variable => true,
                },
            } };
        },
        .@"extern" => {
            const extra = extraData(unwrapped_index.getExtra(ip), Tag.Extern, data);
            const nav = ip.getNav(extra.owner_nav);
            return .{ .@"extern" = .{
                .name = nav.name,
                .ty = extra.ty,
                .lib_name = extra.lib_name,
                .linkage = extra.flags.linkage,
                .visibility = extra.flags.visibility,
                .is_threadlocal = extra.flags.is_threadlocal,
                .is_dll_import = extra.flags.is_dll_import,
                .relocation = extra.flags.relocation,
                .decoration = extra.decoration(),
                .is_const = nav.status.fully_resolved.is_const,
                .alignment = nav.status.fully_resolved.alignment,
                .@"addrspace" = nav.status.fully_resolved.@"addrspace",
                .zir_index = extra.zir_index,
                .owner_nav = extra.owner_nav,
                .source = extra.flags.source,
            } };
        },
        .func_instance => .{ .func = ip.extraFuncInstance(unwrapped_index.tid, unwrapped_index.getExtra(ip), data) },
        .func_decl => .{ .func = extraFuncDecl(unwrapped_index.tid, unwrapped_index.getExtra(ip), data) },
        .func_coerced => .{ .func = ip.extraFuncCoerced(unwrapped_index.getExtra(ip), data) },
        .only_possible_value => {
            const ty: Index = @enumFromInt(data);
            const ty_unwrapped = ty.unwrap(ip);
            const ty_extra = ty_unwrapped.getExtra(ip);
            const ty_item = ty_unwrapped.getItem(ip);
            return switch (ty_item.tag) {
                .type_array_big => {
                    const sentinel = @as(
                        *const [1]Index,
                        @ptrCast(&ty_extra.view().items(.@"0")[ty_item.data + std.meta.fieldIndex(Array, "sentinel").?]),
                    );
                    return .{ .aggregate = .{
                        .ty = ty,
                        .storage = .{ .elems = sentinel[0..@intFromBool(sentinel[0] != .none)] },
                    } };
                },
                .type_array_small,
                .type_vector,
                .type_struct_packed_auto,
                .type_struct_packed_explicit,
                => .{ .aggregate = .{
                    .ty = ty,
                    .storage = .{ .elems = &.{} },
                } },

                // There is only one possible value precisely due to the
                // fact that this values slice is fully populated!
                .type_struct,
                .type_struct_packed_auto_defaults,
                .type_struct_packed_explicit_defaults,
                => {
                    const info = loadStructType(ip, ty);
                    return .{ .aggregate = .{
                        .ty = ty,
                        .storage = .{ .elems = @ptrCast(info.field_defaults.get(ip)) },
                    } };
                },

                // There is only one possible value precisely due to the
                // fact that this values slice is fully populated!
                .type_tuple => {
                    const type_tuple = extraDataTrail(ty_extra, TypeTuple, ty_item.data);
                    const fields_len = type_tuple.data.fields_len;
                    const values = ty_extra.view().items(.@"0")[type_tuple.end + fields_len ..][0..fields_len];
                    return .{ .aggregate = .{
                        .ty = ty,
                        .storage = .{ .elems = @ptrCast(values) },
                    } };
                },

                else => unreachable,
            };
        },
        .bytes => {
            const extra = extraData(unwrapped_index.getExtra(ip), Bytes, data);
            return .{ .aggregate = .{
                .ty = extra.ty,
                .storage = .{ .bytes = extra.bytes },
            } };
        },
        .aggregate => {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, Tag.Aggregate, data);
            const len: u32 = @intCast(ip.aggregateTypeLenIncludingSentinel(extra.data.ty));
            const fields: []const Index = @ptrCast(extra_list.view().items(.@"0")[extra.end..][0..len]);
            return .{ .aggregate = .{
                .ty = extra.data.ty,
                .storage = .{ .elems = fields },
            } };
        },
        .repeated => {
            const extra = extraData(unwrapped_index.getExtra(ip), Repeated, data);
            return .{ .aggregate = .{
                .ty = extra.ty,
                .storage = .{ .repeated_elem = extra.elem_val },
            } };
        },
        .union_value => .{ .un = extraData(unwrapped_index.getExtra(ip), Key.Union, data) },
        .error_set_error => .{ .err = extraData(unwrapped_index.getExtra(ip), Key.Error, data) },
        .error_union_error => {
            const extra = extraData(unwrapped_index.getExtra(ip), Key.Error, data);
            return .{ .error_union = .{
                .ty = extra.ty,
                .val = .{ .err_name = extra.name },
            } };
        },
        .error_union_payload => {
            const extra = extraData(unwrapped_index.getExtra(ip), Tag.TypeValue, data);
            return .{ .error_union = .{
                .ty = extra.ty,
                .val = .{ .payload = extra.val },
            } };
        },
        .enum_literal => .{ .enum_literal = @enumFromInt(data) },
        .enum_tag => .{ .enum_tag = extraData(unwrapped_index.getExtra(ip), Tag.EnumTag, data) },
        .bitpack => .{ .bitpack = extraData(unwrapped_index.getExtra(ip), Key.Bitpack, data) },

        .memoized_call => {
            const extra_list = unwrapped_index.getExtra(ip);
            const extra = extraDataTrail(extra_list, MemoizedCall, data);
            return .{ .memoized_call = .{
                .func = extra.data.func,
                .arg_values = @ptrCast(extra_list.view().items(.@"0")[extra.end..][0..extra.data.args_len]),
                .result = extra.data.result,
                .branch_count = extra.data.branch_count,
            } };
        },
    };
}

fn extraErrorSet(tid: Zcu.PerThread.Id, extra: Local.Extra, extra_index: u32) Key.ErrorSetType {
    const error_set = extraDataTrail(extra, Tag.ErrorSet, extra_index);
    return .{
        .names = .{
            .tid = tid,
            .start = @intCast(error_set.end),
            .len = error_set.data.names_len,
        },
        .names_map = error_set.data.names_map.toOptional(),
    };
}

fn extraTypeTuple(tid: Zcu.PerThread.Id, extra: Local.Extra, extra_index: u32) Key.TupleType {
    const type_tuple = extraDataTrail(extra, TypeTuple, extra_index);
    const fields_len = type_tuple.data.fields_len;
    return .{
        .types = .{
            .tid = tid,
            .start = type_tuple.end,
            .len = fields_len,
        },
        .values = .{
            .tid = tid,
            .start = type_tuple.end + fields_len,
            .len = fields_len,
        },
    };
}

fn extraFuncType(tid: Zcu.PerThread.Id, extra: Local.Extra, extra_index: u32) Key.FuncType {
    const type_function = extraDataTrail(extra, Tag.TypeFunction, extra_index);
    var trail_index: usize = type_function.end;
    const comptime_bits: u32 = if (!type_function.data.flags.has_comptime_bits) 0 else b: {
        const x = extra.view().items(.@"0")[trail_index];
        trail_index += 1;
        break :b x;
    };
    const noalias_bits: u32 = if (!type_function.data.flags.has_noalias_bits) 0 else b: {
        const x = extra.view().items(.@"0")[trail_index];
        trail_index += 1;
        break :b x;
    };
    return .{
        .param_types = .{
            .tid = tid,
            .start = @intCast(trail_index),
            .len = type_function.data.params_len,
        },
        .return_type = type_function.data.return_type,
        .comptime_bits = comptime_bits,
        .noalias_bits = noalias_bits,
        .cc = type_function.data.flags.cc.unpack(),
        .is_var_args = type_function.data.flags.is_var_args,
        .is_noinline = type_function.data.flags.is_noinline,
    };
}

fn extraFuncDecl(tid: Zcu.PerThread.Id, extra: Local.Extra, extra_index: u32) Key.Func {
    const P = Tag.FuncDecl;
    const func_decl = extraDataTrail(extra, P, extra_index);
    return .{
        .tid = tid,
        .ty = func_decl.data.ty,
        .uncoerced_ty = func_decl.data.ty,
        .analysis_extra_index = extra_index + std.meta.fieldIndex(P, "analysis").?,
        .zir_body_inst_extra_index = extra_index + std.meta.fieldIndex(P, "zir_body_inst").?,
        .resolved_error_set_extra_index = if (func_decl.data.analysis.inferred_error_set) func_decl.end else 0,
        .branch_quota_extra_index = 0,
        .owner_nav = func_decl.data.owner_nav,
        .zir_body_inst = func_decl.data.zir_body_inst,
        .lbrace_line = func_decl.data.lbrace_line,
        .rbrace_line = func_decl.data.rbrace_line,
        .lbrace_column = func_decl.data.lbrace_column,
        .rbrace_column = func_decl.data.rbrace_column,
        .generic_owner = .none,
        .comptime_args = Index.Slice.empty,
    };
}

fn extraFuncInstance(ip: *const InternPool, tid: Zcu.PerThread.Id, extra: Local.Extra, extra_index: u32) Key.Func {
    const extra_items = extra.view().items(.@"0");
    const analysis_extra_index = extra_index + std.meta.fieldIndex(Tag.FuncInstance, "analysis").?;
    const analysis: FuncAnalysis = @bitCast(@atomicLoad(u32, &extra_items[analysis_extra_index], .unordered));
    const owner_nav: Nav.Index = @enumFromInt(extra_items[extra_index + std.meta.fieldIndex(Tag.FuncInstance, "owner_nav").?]);
    const ty: Index = @enumFromInt(extra_items[extra_index + std.meta.fieldIndex(Tag.FuncInstance, "ty").?]);
    const generic_owner: Index = @enumFromInt(extra_items[extra_index + std.meta.fieldIndex(Tag.FuncInstance, "generic_owner").?]);
    const func_decl = ip.funcDeclInfo(generic_owner);
    const end_extra_index = extra_index + @as(u32, @typeInfo(Tag.FuncInstance).@"struct".fields.len);
    return .{
        .tid = tid,
        .ty = ty,
        .uncoerced_ty = ty,
        .analysis_extra_index = analysis_extra_index,
        .zir_body_inst_extra_index = func_decl.zir_body_inst_extra_index,
        .resolved_error_set_extra_index = if (analysis.inferred_error_set) end_extra_index else 0,
        .branch_quota_extra_index = extra_index + std.meta.fieldIndex(Tag.FuncInstance, "branch_quota").?,
        .owner_nav = owner_nav,
        .zir_body_inst = func_decl.zir_body_inst,
        .lbrace_line = func_decl.lbrace_line,
        .rbrace_line = func_decl.rbrace_line,
        .lbrace_column = func_decl.lbrace_column,
        .rbrace_column = func_decl.rbrace_column,
        .generic_owner = generic_owner,
        .comptime_args = .{
            .tid = tid,
            .start = end_extra_index + @intFromBool(analysis.inferred_error_set),
            .len = ip.funcTypeParamsLen(func_decl.ty),
        },
    };
}

fn extraFuncCoerced(ip: *const InternPool, extra: Local.Extra, extra_index: u32) Key.Func {
    const func_coerced = extraData(extra, Tag.FuncCoerced, extra_index);
    const func_unwrapped = func_coerced.func.unwrap(ip);
    const sub_item = func_unwrapped.getItem(ip);
    const func_extra = func_unwrapped.getExtra(ip);
    var func: Key.Func = switch (sub_item.tag) {
        .func_instance => ip.extraFuncInstance(func_unwrapped.tid, func_extra, sub_item.data),
        .func_decl => extraFuncDecl(func_unwrapped.tid, func_extra, sub_item.data),
        else => unreachable,
    };
    func.ty = func_coerced.ty;
    return func;
}

fn indexToKeyBigInt(ip: *const InternPool, tid: Zcu.PerThread.Id, limb_index: u32, positive: bool) Key {
    const limbs_items = ip.getLocalShared(tid).getLimbs().view().items(.@"0");
    const int: Int = @bitCast(limbs_items[limb_index..][0..Int.limbs_items_len].*);
    const big_int: BigIntConst = .{
        .limbs = limbs_items[limb_index + Int.limbs_items_len ..][0..int.limbs_len],
        .positive = positive,
    };
    return .{ .int = .{
        .ty = int.ty,
        .storage = if (big_int.toInt(u64)) |x|
            .{ .u64 = x }
        else |_| if (big_int.toInt(i64)) |x|
            .{ .i64 = x }
        else |_|
            .{ .big_int = big_int },
    } };
}

const GetOrPutKey = union(enum) {
    existing: Index,
    new: struct {
        ip: *InternPool,
        tid: Zcu.PerThread.Id,
        io: Io,
        shard: *Shard,
        map_index: u32,
    },

    fn put(gop: *GetOrPutKey) Index {
        switch (gop.*) {
            .existing => unreachable,
            .new => |*info| {
                const index = Index.Unwrapped.wrap(.{
                    .tid = info.tid,
                    .index = info.ip.getLocal(info.tid).mutate.items.len - 1,
                }, info.ip);
                gop.putTentative(index);
                gop.putFinal(index);
                return index;
            },
        }
    }

    fn putTentative(gop: *GetOrPutKey, index: Index) void {
        assert(index != .none);
        switch (gop.*) {
            .existing => unreachable,
            .new => |*info| gop.new.shard.shared.map.entries[info.map_index].release(index),
        }
    }

    fn putFinal(gop: *GetOrPutKey, index: Index) void {
        assert(index != .none);
        switch (gop.*) {
            .existing => unreachable,
            .new => |info| {
                assert(info.shard.shared.map.entries[info.map_index].value == index);
                info.shard.mutate.map.len += 1;
                info.shard.mutate.map.mutex.unlock(info.io);
                gop.* = .{ .existing = index };
            },
        }
    }

    fn cancel(gop: *GetOrPutKey) void {
        switch (gop.*) {
            .existing => {},
            .new => |info| info.shard.mutate.map.mutex.unlock(info.io),
        }
        gop.* = .{ .existing = undefined };
    }

    fn deinit(gop: *GetOrPutKey) void {
        switch (gop.*) {
            .existing => {},
            .new => |info| info.shard.shared.map.entries[info.map_index].resetUnordered(),
        }
        gop.cancel();
        gop.* = undefined;
    }
};
fn getOrPutKey(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    key: Key,
) Allocator.Error!GetOrPutKey {
    return ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, key, 0);
}
fn getOrPutKeyEnsuringAdditionalCapacity(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    key: Key,
    additional_capacity: u32,
) Allocator.Error!GetOrPutKey {
    const full_hash = key.hash64(ip);
    const hash: u32 = @truncate(full_hash >> 32);
    const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
    var map = shard.shared.map.acquire();
    const Map = @TypeOf(map);
    var map_mask = map.header().mask();
    var map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.acquire();
        if (index == .none) break;
        if (entry.hash != hash) continue;
        if (index.unwrap(ip).getTag(ip) == .removed) continue;
        if (ip.indexToKey(index).eql(key, ip)) return .{ .existing = index };
    }
    shard.mutate.map.mutex.lock(io, tid);
    errdefer shard.mutate.map.mutex.unlock(io);
    if (map.entries != shard.shared.map.entries) {
        map = shard.shared.map;
        map_mask = map.header().mask();
        map_index = hash;
    }
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.value;
        if (index == .none) break;
        if (entry.hash != hash) continue;
        if (ip.indexToKey(index).eql(key, ip)) {
            defer shard.mutate.map.mutex.unlock(io);
            return .{ .existing = index };
        }
    }
    const map_header = map.header().*;
    const required = shard.mutate.map.len + additional_capacity;
    if (required >= map_header.capacity * 3 / 5) {
        const arena_state = &ip.getLocal(tid).mutate.arena;
        var arena = arena_state.promote(gpa);
        defer arena_state.* = arena.state;
        var new_map_capacity = map_header.capacity;
        while (true) {
            new_map_capacity *= 2;
            if (required < new_map_capacity * 3 / 5) break;
        }
        const new_map_buf = try arena.allocator().alignedAlloc(
            u8,
            .fromByteUnits(Map.alignment),
            Map.entries_offset + new_map_capacity * @sizeOf(Map.Entry),
        );
        const new_map: Map = .{ .entries = @ptrCast(new_map_buf[Map.entries_offset..].ptr) };
        new_map.header().* = .{ .capacity = new_map_capacity };
        @memset(new_map.entries[0..new_map_capacity], .{ .value = .none, .hash = undefined });
        const new_map_mask = new_map.header().mask();
        map_index = 0;
        while (map_index < map_header.capacity) : (map_index += 1) {
            const entry = &map.entries[map_index];
            const index = entry.value;
            if (index == .none) continue;
            const item_hash = entry.hash;
            var new_map_index = item_hash;
            while (true) : (new_map_index += 1) {
                new_map_index &= new_map_mask;
                const new_entry = &new_map.entries[new_map_index];
                if (new_entry.value != .none) continue;
                new_entry.* = .{
                    .value = index,
                    .hash = item_hash,
                };
                break;
            }
        }
        map = new_map;
        map_index = hash;
        while (true) : (map_index += 1) {
            map_index &= new_map_mask;
            if (map.entries[map_index].value == .none) break;
        }
        shard.shared.map.release(new_map);
    }
    map.entries[map_index].hash = hash;
    return .{ .new = .{
        .ip = ip,
        .tid = tid,
        .io = io,
        .shard = shard,
        .map_index = map_index,
    } };
}

pub fn get(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, key: Key) Allocator.Error!Index {
    var gop = try ip.getOrPutKey(gpa, io, tid, key);
    defer gop.deinit();
    if (gop == .existing) return gop.existing;
    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);
    switch (key) {
        .int_type => |int_type| {
            const t: Tag = switch (int_type.signedness) {
                .signed => .type_int_signed,
                .unsigned => .type_int_unsigned,
            };
            items.appendAssumeCapacity(.{
                .tag = t,
                .data = int_type.bits,
            });
        },
        .ptr_type => |ptr_type| {
            assert(ptr_type.child != .none);
            assert(ptr_type.sentinel == .none or ip.typeOf(ptr_type.sentinel) == ptr_type.child);

            if (ptr_type.flags.size == .slice) {
                gop.cancel();
                var new_key = key;
                new_key.ptr_type.flags.size = .many;
                const ptr_type_index = try ip.get(gpa, io, tid, new_key);
                gop = try ip.getOrPutKey(gpa, io, tid, key);

                try items.ensureUnusedCapacity(1);
                items.appendAssumeCapacity(.{
                    .tag = .type_slice,
                    .data = @intFromEnum(ptr_type_index),
                });
                return gop.put();
            }

            var ptr_type_adjusted = ptr_type;
            if (ptr_type.flags.size == .c) ptr_type_adjusted.flags.is_allowzero = true;

            items.appendAssumeCapacity(.{
                .tag = .type_pointer,
                .data = try addExtra(extra, ptr_type_adjusted),
            });
        },
        .array_type => |array_type| {
            assert(array_type.child != .none);
            assert(array_type.sentinel == .none or ip.typeOf(array_type.sentinel) == array_type.child);

            if (std.math.cast(u32, array_type.len)) |len| {
                if (array_type.sentinel == .none) {
                    items.appendAssumeCapacity(.{
                        .tag = .type_array_small,
                        .data = try addExtra(extra, Vector{
                            .len = len,
                            .child = array_type.child,
                        }),
                    });
                    return gop.put();
                }
            }

            const length = Array.Length.init(array_type.len);
            items.appendAssumeCapacity(.{
                .tag = .type_array_big,
                .data = try addExtra(extra, Array{
                    .len0 = length.a,
                    .len1 = length.b,
                    .child = array_type.child,
                    .sentinel = array_type.sentinel,
                }),
            });
        },
        .vector_type => |vector_type| {
            items.appendAssumeCapacity(.{
                .tag = .type_vector,
                .data = try addExtra(extra, Vector{
                    .len = vector_type.len,
                    .child = vector_type.child,
                }),
            });
        },
        .opt_type => |payload_type| {
            assert(payload_type != .none);
            items.appendAssumeCapacity(.{
                .tag = .type_optional,
                .data = @intFromEnum(payload_type),
            });
        },
        .anyframe_type => |payload_type| {
            // payload_type might be none, indicating the type is `anyframe`.
            items.appendAssumeCapacity(.{
                .tag = .type_anyframe,
                .data = @intFromEnum(payload_type),
            });
        },
        .error_union_type => |error_union_type| {
            items.appendAssumeCapacity(if (error_union_type.error_set_type == .anyerror_type) .{
                .tag = .type_anyerror_union,
                .data = @intFromEnum(error_union_type.payload_type),
            } else .{
                .tag = .type_error_union,
                .data = try addExtra(extra, error_union_type),
            });
        },
        .error_set_type => |error_set_type| {
            assert(error_set_type.names_map == .none);
            assert(std.sort.isSorted(NullTerminatedString, error_set_type.names.get(ip), {}, NullTerminatedString.indexLessThan));
            const names = error_set_type.names.get(ip);
            const names_map = try ip.addMap(gpa, io, tid, names.len);
            ip.addStringsToMap(names_map, names);
            const names_len = error_set_type.names.len;
            try extra.ensureUnusedCapacity(@typeInfo(Tag.ErrorSet).@"struct".fields.len + names_len);
            items.appendAssumeCapacity(.{
                .tag = .type_error_set,
                .data = addExtraAssumeCapacity(extra, Tag.ErrorSet{
                    .names_len = names_len,
                    .names_map = names_map,
                }),
            });
            extra.appendSliceAssumeCapacity(.{@ptrCast(error_set_type.names.get(ip))});
        },
        .inferred_error_set_type => |ies_index| {
            items.appendAssumeCapacity(.{
                .tag = .type_inferred_error_set,
                .data = @intFromEnum(ies_index),
            });
        },
        .simple_type => |simple_type| {
            assert(@intFromEnum(simple_type) == items.mutate.len);
            items.appendAssumeCapacity(.{
                .tag = .simple_type,
                .data = 0, // avoid writing `undefined` bits to a file
            });
        },
        .simple_value => |simple_value| {
            assert(@intFromEnum(simple_value) == items.mutate.len);
            items.appendAssumeCapacity(.{
                .tag = .simple_value,
                .data = 0, // avoid writing `undefined` bits to a file
            });
        },
        .undef => |ty| {
            assert(ty != .none);
            items.appendAssumeCapacity(.{
                .tag = .undef,
                .data = @intFromEnum(ty),
            });
        },

        .struct_type => unreachable, // instead use: getDeclaredStructType, getReifiedStructType
        .union_type => unreachable, // instead use: getDeclaredUnionType, getReifiedUnionType
        .enum_type => unreachable, // instead use: getDeclaredEnumType, getReifiedEnumType, getGeneratedEnumTagType
        .opaque_type => unreachable, // instead use: getDeclaredOpaqueType

        .tuple_type => unreachable, // use getTupleType() instead
        .func_type => unreachable, // use getFuncType() instead
        .@"extern" => unreachable, // use getExtern() instead
        .func => unreachable, // use getFuncInstance() or getFuncDecl() instead
        .un => unreachable, // use getUnion instead

        .variable => |variable| {
            const has_init = variable.init != .none;
            if (has_init) assert(variable.ty == ip.typeOf(variable.init));
            items.appendAssumeCapacity(.{
                .tag = switch (variable.is_threadlocal) {
                    false => .variable,
                    true => .threadlocal_variable,
                },
                .data = try addExtra(extra, Tag.Variable{
                    .ty = variable.ty,
                    .init = variable.init,
                    .owner_nav = variable.owner_nav,
                }),
            });
        },

        .slice => |slice| {
            assert(ip.indexToKey(slice.ty).ptr_type.flags.size == .slice);
            assert(ip.indexToKey(ip.typeOf(slice.ptr)).ptr_type.flags.size == .many);
            items.appendAssumeCapacity(.{
                .tag = .ptr_slice,
                .data = try addExtra(extra, PtrSlice{
                    .ty = slice.ty,
                    .ptr = slice.ptr,
                    .len = slice.len,
                }),
            });
        },

        .ptr => |ptr| {
            const ptr_type = ip.indexToKey(ptr.ty).ptr_type;
            assert(ptr_type.flags.size != .slice);
            items.appendAssumeCapacity(switch (ptr.base_addr) {
                .nav => |nav| .{
                    .tag = .ptr_nav,
                    .data = try addExtra(extra, PtrNav.init(ptr.ty, nav, ptr.byte_offset)),
                },
                .comptime_alloc => |alloc_index| .{
                    .tag = .ptr_comptime_alloc,
                    .data = try addExtra(extra, PtrComptimeAlloc.init(ptr.ty, alloc_index, ptr.byte_offset)),
                },
                .uav => |uav| if (ptrsHaveSameAlignment(ip, ptr.ty, ptr_type, uav.orig_ty)) item: {
                    if (ptr.ty != uav.orig_ty) {
                        gop.cancel();
                        var new_key = key;
                        new_key.ptr.base_addr.uav.orig_ty = ptr.ty;
                        gop = try ip.getOrPutKey(gpa, io, tid, new_key);
                        if (gop == .existing) return gop.existing;
                    }
                    break :item .{
                        .tag = .ptr_uav,
                        .data = try addExtra(extra, PtrUav.init(ptr.ty, uav.val, ptr.byte_offset)),
                    };
                } else .{
                    .tag = .ptr_uav_aligned,
                    .data = try addExtra(extra, PtrUavAligned.init(ptr.ty, uav.val, uav.orig_ty, ptr.byte_offset)),
                },
                .comptime_field => |field_val| item: {
                    assert(field_val != .none);
                    break :item .{
                        .tag = .ptr_comptime_field,
                        .data = try addExtra(extra, PtrComptimeField.init(ptr.ty, field_val, ptr.byte_offset)),
                    };
                },
                .eu_payload, .opt_payload => |base| item: {
                    switch (ptr.base_addr) {
                        .eu_payload => assert(ip.indexToKey(
                            ip.indexToKey(ip.typeOf(base)).ptr_type.child,
                        ) == .error_union_type),
                        .opt_payload => assert(ip.indexToKey(
                            ip.indexToKey(ip.typeOf(base)).ptr_type.child,
                        ) == .opt_type),
                        else => unreachable,
                    }
                    break :item .{
                        .tag = switch (ptr.base_addr) {
                            .eu_payload => .ptr_eu_payload,
                            .opt_payload => .ptr_opt_payload,
                            else => unreachable,
                        },
                        .data = try addExtra(extra, PtrBase.init(ptr.ty, base, ptr.byte_offset)),
                    };
                },
                .int => .{
                    .tag = .ptr_int,
                    .data = try addExtra(extra, PtrInt.init(ptr.ty, ptr.byte_offset)),
                },
                .arr_elem, .field => |base_index| {
                    const base_ptr_type = ip.indexToKey(ip.typeOf(base_index.base)).ptr_type;
                    switch (ptr.base_addr) {
                        .arr_elem => assert(base_ptr_type.flags.size == .many),
                        .field => {
                            assert(base_ptr_type.flags.size == .one);
                            switch (ip.indexToKey(base_ptr_type.child)) {
                                .tuple_type => |tuple_type| {
                                    assert(ptr.base_addr == .field);
                                    assert(base_index.index < tuple_type.types.len);
                                },
                                .struct_type => {
                                    assert(ptr.base_addr == .field);
                                    assert(base_index.index < ip.loadStructType(base_ptr_type.child).field_types.len);
                                },
                                .union_type => {
                                    const union_type = ip.loadUnionType(base_ptr_type.child);
                                    assert(ptr.base_addr == .field);
                                    assert(base_index.index < union_type.field_types.len);
                                },
                                .ptr_type => |slice_type| {
                                    assert(ptr.base_addr == .field);
                                    assert(slice_type.flags.size == .slice);
                                    assert(base_index.index < 2);
                                },
                                else => unreachable,
                            }
                        },
                        else => unreachable,
                    }
                    gop.cancel();
                    const index_index = try ip.get(gpa, io, tid, .{ .int = .{
                        .ty = .usize_type,
                        .storage = .{ .u64 = base_index.index },
                    } });
                    gop = try ip.getOrPutKey(gpa, io, tid, key);
                    try items.ensureUnusedCapacity(1);
                    items.appendAssumeCapacity(.{
                        .tag = switch (ptr.base_addr) {
                            .arr_elem => .ptr_elem,
                            .field => .ptr_field,
                            else => unreachable,
                        },
                        .data = try addExtra(extra, PtrBaseIndex.init(ptr.ty, base_index.base, index_index, ptr.byte_offset)),
                    });
                    return gop.put();
                },
            });
        },

        .opt => |opt| {
            assert(ip.isOptionalType(opt.ty));
            assert(opt.val == .none or ip.indexToKey(opt.ty).opt_type == ip.typeOf(opt.val));
            items.appendAssumeCapacity(if (opt.val == .none) .{
                .tag = .opt_null,
                .data = @intFromEnum(opt.ty),
            } else .{
                .tag = .opt_payload,
                .data = try addExtra(extra, Tag.TypeValue{
                    .ty = opt.ty,
                    .val = opt.val,
                }),
            });
        },

        .int => |int| b: {
            assert(ip.isIntegerType(int.ty));
            switch (int.ty) {
                .u8_type => switch (int.storage) {
                    .big_int => |big_int| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_u8,
                            .data = big_int.toInt(u8) catch unreachable,
                        });
                        break :b;
                    },
                    inline .u64, .i64 => |x| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_u8,
                            .data = @as(u8, @intCast(x)),
                        });
                        break :b;
                    },
                },
                .u16_type => switch (int.storage) {
                    .big_int => |big_int| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_u16,
                            .data = big_int.toInt(u16) catch unreachable,
                        });
                        break :b;
                    },
                    inline .u64, .i64 => |x| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_u16,
                            .data = @as(u16, @intCast(x)),
                        });
                        break :b;
                    },
                },
                .u32_type => switch (int.storage) {
                    .big_int => |big_int| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_u32,
                            .data = big_int.toInt(u32) catch unreachable,
                        });
                        break :b;
                    },
                    inline .u64, .i64 => |x| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_u32,
                            .data = @as(u32, @intCast(x)),
                        });
                        break :b;
                    },
                },
                .i32_type => switch (int.storage) {
                    .big_int => |big_int| {
                        const casted = big_int.toInt(i32) catch unreachable;
                        items.appendAssumeCapacity(.{
                            .tag = .int_i32,
                            .data = @as(u32, @bitCast(casted)),
                        });
                        break :b;
                    },
                    inline .u64, .i64 => |x| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_i32,
                            .data = @as(u32, @bitCast(@as(i32, @intCast(x)))),
                        });
                        break :b;
                    },
                },
                .usize_type => switch (int.storage) {
                    .big_int => |big_int| {
                        if (big_int.toInt(u32)) |casted| {
                            items.appendAssumeCapacity(.{
                                .tag = .int_usize,
                                .data = casted,
                            });
                            break :b;
                        } else |_| {}
                    },
                    inline .u64, .i64 => |x| {
                        if (std.math.cast(u32, x)) |casted| {
                            items.appendAssumeCapacity(.{
                                .tag = .int_usize,
                                .data = casted,
                            });
                            break :b;
                        }
                    },
                },
                .comptime_int_type => switch (int.storage) {
                    .big_int => |big_int| {
                        if (big_int.toInt(u32)) |casted| {
                            items.appendAssumeCapacity(.{
                                .tag = .int_comptime_int_u32,
                                .data = casted,
                            });
                            break :b;
                        } else |_| {}
                        if (big_int.toInt(i32)) |casted| {
                            items.appendAssumeCapacity(.{
                                .tag = .int_comptime_int_i32,
                                .data = @as(u32, @bitCast(casted)),
                            });
                            break :b;
                        } else |_| {}
                    },
                    inline .u64, .i64 => |x| {
                        if (std.math.cast(u32, x)) |casted| {
                            items.appendAssumeCapacity(.{
                                .tag = .int_comptime_int_u32,
                                .data = casted,
                            });
                            break :b;
                        }
                        if (std.math.cast(i32, x)) |casted| {
                            items.appendAssumeCapacity(.{
                                .tag = .int_comptime_int_i32,
                                .data = @as(u32, @bitCast(casted)),
                            });
                            break :b;
                        }
                    },
                },
                else => {},
            }
            switch (int.storage) {
                .big_int => |big_int| {
                    if (big_int.toInt(u32)) |casted| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_small,
                            .data = try addExtra(extra, IntSmall{
                                .ty = int.ty,
                                .value = casted,
                            }),
                        });
                        return gop.put();
                    } else |_| {}

                    const tag: Tag = if (big_int.positive) .int_positive else .int_negative;
                    try addInt(ip, gpa, io, tid, int.ty, tag, big_int.limbs);
                },
                inline .u64, .i64 => |x| {
                    if (std.math.cast(u32, x)) |casted| {
                        items.appendAssumeCapacity(.{
                            .tag = .int_small,
                            .data = try addExtra(extra, IntSmall{
                                .ty = int.ty,
                                .value = casted,
                            }),
                        });
                        return gop.put();
                    }

                    var buf: [2]Limb = undefined;
                    const big_int = BigIntMutable.init(&buf, x).toConst();
                    const tag: Tag = if (big_int.positive) .int_positive else .int_negative;
                    try addInt(ip, gpa, io, tid, int.ty, tag, big_int.limbs);
                },
            }
        },

        .err => |err| {
            assert(ip.isErrorSetType(err.ty));
            items.appendAssumeCapacity(.{
                .tag = .error_set_error,
                .data = try addExtra(extra, err),
            });
        },

        .error_union => |error_union| {
            assert(ip.isErrorUnionType(error_union.ty));
            items.appendAssumeCapacity(switch (error_union.val) {
                .err_name => |err_name| .{
                    .tag = .error_union_error,
                    .data = try addExtra(extra, Key.Error{
                        .ty = error_union.ty,
                        .name = err_name,
                    }),
                },
                .payload => |payload| .{
                    .tag = .error_union_payload,
                    .data = try addExtra(extra, Tag.TypeValue{
                        .ty = error_union.ty,
                        .val = payload,
                    }),
                },
            });
        },

        .enum_literal => |enum_literal| items.appendAssumeCapacity(.{
            .tag = .enum_literal,
            .data = @intFromEnum(enum_literal),
        }),

        .enum_tag => |enum_tag| {
            assert(ip.isEnumType(enum_tag.ty));
            switch (ip.indexToKey(enum_tag.ty)) {
                .simple_type => assert(ip.isIntegerType(ip.typeOf(enum_tag.int))),
                .enum_type => assert(ip.typeOf(enum_tag.int) == ip.loadEnumType(enum_tag.ty).int_tag_type),
                else => unreachable,
            }
            items.appendAssumeCapacity(.{
                .tag = .enum_tag,
                .data = try addExtra(extra, enum_tag),
            });
        },

        .float => |float| {
            switch (float.ty) {
                .f16_type => items.appendAssumeCapacity(.{
                    .tag = .float_f16,
                    .data = @as(u16, @bitCast(float.storage.f16)),
                }),
                .f32_type => items.appendAssumeCapacity(.{
                    .tag = .float_f32,
                    .data = @as(u32, @bitCast(float.storage.f32)),
                }),
                .f64_type => items.appendAssumeCapacity(.{
                    .tag = .float_f64,
                    .data = try addExtra(extra, Float64.pack(float.storage.f64)),
                }),
                .f80_type => items.appendAssumeCapacity(.{
                    .tag = .float_f80,
                    .data = try addExtra(extra, Float80.pack(float.storage.f80)),
                }),
                .f128_type => items.appendAssumeCapacity(.{
                    .tag = .float_f128,
                    .data = try addExtra(extra, Float128.pack(float.storage.f128)),
                }),
                .c_longdouble_type => switch (float.storage) {
                    .f80 => |x| items.appendAssumeCapacity(.{
                        .tag = .float_c_longdouble_f80,
                        .data = try addExtra(extra, Float80.pack(x)),
                    }),
                    inline .f16, .f32, .f64, .f128 => |x| items.appendAssumeCapacity(.{
                        .tag = .float_c_longdouble_f128,
                        .data = try addExtra(extra, Float128.pack(x)),
                    }),
                },
                .comptime_float_type => items.appendAssumeCapacity(.{
                    .tag = .float_comptime_float,
                    .data = try addExtra(extra, Float128.pack(float.storage.f128)),
                }),
                else => unreachable,
            }
        },

        .aggregate => |aggregate| {
            const ty_key = ip.indexToKey(aggregate.ty);
            const len = ip.aggregateTypeLen(aggregate.ty);
            const child: Index, const sentinel: Index = switch (ty_key) {
                .array_type => |array_type| .{ array_type.child, array_type.sentinel },
                .vector_type => |vector_type| .{ vector_type.child, .none },
                .tuple_type => .{ .none, .none },
                .struct_type => child: {
                    assert(ip.loadStructType(aggregate.ty).layout != .@"packed");
                    break :child .{ .none, .none };
                },
                else => unreachable,
            };
            const len_including_sentinel = len + @intFromBool(sentinel != .none);
            switch (aggregate.storage) {
                .bytes => |bytes| {
                    assert(child == .u8_type);
                    if (sentinel != .none) {
                        assert(bytes.at(@intCast(len), ip) == ip.indexToKey(sentinel).int.storage.u64);
                    }
                },
                .elems => |elems| {
                    if (elems.len != len) {
                        assert(elems.len == len_including_sentinel);
                        assert(elems[@intCast(len)] == sentinel);
                    }
                },
                .repeated_elem => |elem| {
                    assert(sentinel == .none or elem == sentinel);
                },
            }
            if (aggregate.storage.values().len > 0) switch (ty_key) {
                .array_type, .vector_type => {
                    var any_defined = false;
                    for (aggregate.storage.values()) |elem| {
                        if (!ip.isUndef(elem)) any_defined = true;
                        assert(ip.typeOf(elem) == child);
                    }
                    assert(any_defined); // aggregate fields must not be all undefined
                },
                .struct_type => {
                    var any_defined = false;
                    for (aggregate.storage.values(), ip.loadStructType(aggregate.ty).field_types.get(ip)) |elem, field_ty| {
                        if (!ip.isUndef(elem)) any_defined = true;
                        assert(ip.typeOf(elem) == field_ty);
                    }
                    assert(any_defined); // aggregate fields must not be all undefined
                },
                .tuple_type => |tuple_type| {
                    var any_defined = false;
                    for (aggregate.storage.values(), tuple_type.types.get(ip)) |elem, ty| {
                        if (!ip.isUndef(elem)) any_defined = true;
                        assert(ip.typeOf(elem) == ty);
                    }
                    assert(any_defined); // aggregate fields must not be all undefined
                },
                else => unreachable,
            };

            if (len == 0) {
                items.appendAssumeCapacity(.{
                    .tag = .only_possible_value,
                    .data = @intFromEnum(aggregate.ty),
                });
                return gop.put();
            }

            switch (ty_key) {
                .tuple_type => |tuple_type| opv: {
                    switch (aggregate.storage) {
                        .bytes => |bytes| for (tuple_type.values.get(ip), bytes.at(0, ip)..) |value, byte| {
                            if (value == .none) break :opv;
                            switch (ip.indexToKey(value)) {
                                .undef => break :opv,
                                .int => |int| switch (int.storage) {
                                    .u64 => |x| if (x != byte) break :opv,
                                    else => break :opv,
                                },
                                else => unreachable,
                            }
                        },
                        .elems => |elems| if (!std.mem.eql(
                            Index,
                            tuple_type.values.get(ip),
                            elems,
                        )) break :opv,
                        .repeated_elem => |elem| for (tuple_type.values.get(ip)) |value| {
                            if (value != elem) break :opv;
                        },
                    }
                    // This encoding works thanks to the fact that, as we just verified,
                    // the type itself contains a slice of values that can be provided
                    // in the aggregate fields.
                    items.appendAssumeCapacity(.{
                        .tag = .only_possible_value,
                        .data = @intFromEnum(aggregate.ty),
                    });
                    return gop.put();
                },
                else => {},
            }

            repeated: {
                switch (aggregate.storage) {
                    .bytes => |bytes| for (bytes.toSlice(len, ip)[1..]) |byte|
                        if (byte != bytes.at(0, ip)) break :repeated,
                    .elems => |elems| for (elems[1..@intCast(len)]) |elem|
                        if (elem != elems[0]) break :repeated,
                    .repeated_elem => {},
                }
                const elem = switch (aggregate.storage) {
                    .bytes => |bytes| elem: {
                        gop.cancel();
                        const elem = try ip.get(gpa, io, tid, .{ .int = .{
                            .ty = .u8_type,
                            .storage = .{ .u64 = bytes.at(0, ip) },
                        } });
                        gop = try ip.getOrPutKey(gpa, io, tid, key);
                        try items.ensureUnusedCapacity(1);
                        break :elem elem;
                    },
                    .elems => |elems| elems[0],
                    .repeated_elem => |elem| elem,
                };

                try extra.ensureUnusedCapacity(@typeInfo(Repeated).@"struct".fields.len);
                items.appendAssumeCapacity(.{
                    .tag = .repeated,
                    .data = addExtraAssumeCapacity(extra, Repeated{
                        .ty = aggregate.ty,
                        .elem_val = elem,
                    }),
                });
                return gop.put();
            }

            if (child == .u8_type) bytes: {
                const string_bytes = ip.getLocal(tid).getMutableStringBytes(gpa, io);
                const start = string_bytes.mutate.len;
                try string_bytes.ensureUnusedCapacity(@intCast(len_including_sentinel + 1));
                try extra.ensureUnusedCapacity(@typeInfo(Bytes).@"struct".fields.len);
                switch (aggregate.storage) {
                    .bytes => |bytes| string_bytes.appendSliceAssumeCapacity(.{bytes.toSlice(len, ip)}),
                    .elems => |elems| for (elems[0..@intCast(len)]) |elem| switch (ip.indexToKey(elem)) {
                        .undef => {
                            string_bytes.shrinkRetainingCapacity(start);
                            break :bytes;
                        },
                        .int => |int| string_bytes.appendAssumeCapacity(.{@intCast(int.storage.u64)}),
                        else => unreachable,
                    },
                    .repeated_elem => |elem| switch (ip.indexToKey(elem)) {
                        .undef => break :bytes,
                        .int => |int| @memset(
                            string_bytes.addManyAsSliceAssumeCapacity(@intCast(len))[0],
                            @intCast(int.storage.u64),
                        ),
                        else => unreachable,
                    },
                }
                if (sentinel != .none) string_bytes.appendAssumeCapacity(.{
                    @intCast(ip.indexToKey(sentinel).int.storage.u64),
                });
                const string = try ip.getOrPutTrailingString(
                    gpa,
                    io,
                    tid,
                    @intCast(len_including_sentinel),
                    .maybe_embedded_nulls,
                );
                items.appendAssumeCapacity(.{
                    .tag = .bytes,
                    .data = addExtraAssumeCapacity(extra, Bytes{
                        .ty = aggregate.ty,
                        .bytes = string,
                    }),
                });
                return gop.put();
            }

            try extra.ensureUnusedCapacity(
                @typeInfo(Tag.Aggregate).@"struct".fields.len + @as(usize, @intCast(len_including_sentinel + 1)),
            );
            items.appendAssumeCapacity(.{
                .tag = .aggregate,
                .data = addExtraAssumeCapacity(extra, Tag.Aggregate{
                    .ty = aggregate.ty,
                }),
            });
            extra.appendSliceAssumeCapacity(.{@ptrCast(aggregate.storage.elems)});
            if (sentinel != .none) extra.appendAssumeCapacity(.{@intFromEnum(sentinel)});
        },
        .bitpack => |bitpack| {
            switch (ip.zigTypeTag(bitpack.ty)) {
                .@"struct" => assert(ip.typeOf(bitpack.backing_int_val) == ip.loadStructType(bitpack.ty).packed_backing_int_type),
                .@"union" => assert(ip.typeOf(bitpack.backing_int_val) == ip.loadUnionType(bitpack.ty).packed_backing_int_type),
                else => unreachable,
            }
            assert(!ip.isUndef(bitpack.backing_int_val));
            items.appendAssumeCapacity(.{
                .tag = .bitpack,
                .data = try addExtra(extra, bitpack),
            });
        },

        .memoized_call => |memoized_call| {
            for (memoized_call.arg_values) |arg| assert(arg != .none);
            try extra.ensureUnusedCapacity(@typeInfo(MemoizedCall).@"struct".fields.len +
                memoized_call.arg_values.len);
            items.appendAssumeCapacity(.{
                .tag = .memoized_call,
                .data = addExtraAssumeCapacity(extra, MemoizedCall{
                    .func = memoized_call.func,
                    .args_len = @intCast(memoized_call.arg_values.len),
                    .result = memoized_call.result,
                    .branch_count = memoized_call.branch_count,
                }),
            });
            extra.appendSliceAssumeCapacity(.{@ptrCast(memoized_call.arg_values)});
        },
    }
    return gop.put();
}

pub fn getDeclaredStructType(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    ini: struct {
        zir_index: TrackedInst.Index,
        captures: []const CaptureValue,

        // If the value of any of the following fields would change on an incremental update, then logic
        // in `Zcu.mapOldZirToNew` must detect that (these properties are all trivially known from ZIR)
        // and refuse to map the type declaration. This causes `zir_index` to change so that a new type
        // will be interned at a fresh index.
        //
        // In the future, it would be good to remove all of those fields from `ini`, and in fact just
        // have a single function `getDeclaredContainer` which is suitable for all container types.
        // However, this requires some major changes to how container types are represented in the
        // InternPool, so that it is possible for their backing storage to be "reallocated" as needed
        // during type resolution.
        fields_len: u32,
        layout: std.builtin.Type.ContainerLayout,
        any_comptime_fields: bool,
        any_field_defaults: bool,
        any_field_aligns: bool,
        packed_backing_mode: BackingTypeMode,
    },
) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .struct_type = .{ .declared = .{
        .zir_index = ini.zir_index,
        .captures = .{ .external = ini.captures },
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const field_name_map = try ip.addMap(gpa, io, tid, ini.fields_len);
    errdefer local.mutate.maps.len -= 1;

    const is_extern = switch (ini.layout) {
        .auto => false,
        .@"extern" => true,
        .@"packed" => {
            try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeStructPacked).@"struct".fields.len +
                ini.captures.len + // capture
                ini.fields_len + // field_name
                ini.fields_len + // field_type
                (if (ini.any_field_defaults) ini.fields_len else 0)); // field_default

            const extra_index = addExtraAssumeCapacity(extra, Tag.TypeStructPacked{
                .zir_index = ini.zir_index,
                .bits = .{
                    .captures_len = @enumFromInt(ini.captures.len),
                    .want_layout = false,
                },
                .name = undefined, // set by `finish`
                .name_nav = undefined, // set by `finish`
                .namespace = undefined, // set by `finish`
                .backing_int_type = .none,
                .fields_len = ini.fields_len,
                .field_name_map = field_name_map,
            });
            extra.appendSliceAssumeCapacity(.{@ptrCast(ini.captures)}); // capture
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
            if (ini.any_field_defaults) {
                extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_default
            }
            items.appendAssumeCapacity(.{
                .tag = switch (ini.packed_backing_mode) {
                    .auto => if (ini.any_field_defaults) .type_struct_packed_auto_defaults else .type_struct_packed_auto,
                    .explicit => if (ini.any_field_defaults) .type_struct_packed_explicit_defaults else .type_struct_packed_explicit,
                },
                .data = extra_index,
            });
            return .{ .wip = .{
                .index = gop.put(),
                .tid = tid,
                .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeStructPacked, "name").?,
                .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeStructPacked, "name_nav").?,
                .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeStructPacked, "namespace").?,
                .field_names = undefined,
                .field_types = undefined,
                .field_values = undefined,
                .field_aligns = undefined,
                .field_is_comptime_bits = undefined,
            } };
        },
    };

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeStruct).@"struct".fields.len +
        1 + // captures_len
        ini.captures.len + // capture
        ini.fields_len + // field_name
        ini.fields_len + // field_type
        (if (ini.any_field_defaults) ini.fields_len else 0) + // field_default
        (if (ini.any_field_aligns) (ini.fields_len + 3) / 4 else 0) + // field_align
        (if (ini.any_comptime_fields) (ini.fields_len + 31) / 32 else 0) + // field_is_comptime_bits
        (if (!is_extern) ini.fields_len else 0) + // field_runtime_order
        ini.fields_len); // field_offset

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeStruct{
        .zir_index = ini.zir_index,
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .fields_len = ini.fields_len,
        .field_name_map = field_name_map,
        .size = 0,
        .flags = .{
            .any_captures = if (ini.captures.len != 0) .true else .false,
            .layout = if (is_extern) .@"extern" else .auto,
            .any_comptime_fields = ini.any_comptime_fields,
            .any_field_defaults = ini.any_field_defaults,
            .any_field_aligns = ini.any_field_aligns,
            .class = .no_possible_value,
            .alignment = .none,
            .want_layout = false,
        },
    });
    if (ini.captures.len != 0) {
        extra.appendAssumeCapacity(.{@intCast(ini.captures.len)}); // captures_len
        extra.appendSliceAssumeCapacity(.{@ptrCast(ini.captures)}); // capture
    }
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
    if (ini.any_field_defaults) {
        extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_default
    }
    if (ini.any_field_aligns) {
        extra.appendNTimesAssumeCapacity(.{0}, (ini.fields_len + 3) / 4); // field_align
    }
    if (ini.any_comptime_fields) {
        extra.appendNTimesAssumeCapacity(.{0}, (ini.fields_len + 31) / 32); // field_is_comptime_bits
    }
    if (!is_extern) {
        extra.appendNTimesAssumeCapacity(.{@intFromEnum(LoadedStructType.RuntimeOrder.unresolved)}, ini.fields_len); // field_runtime_order
    }
    extra.appendNTimesAssumeCapacity(.{0}, ini.fields_len); // field_offset
    items.appendAssumeCapacity(.{
        .tag = .type_struct,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeStruct, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeStruct, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeStruct, "namespace").?,
        .field_names = undefined,
        .field_types = undefined,
        .field_values = undefined,
        .field_aligns = undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub fn getReifiedStructType(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, ini: struct {
    zir_index: TrackedInst.Index,
    type_hash: u64,
    fields_len: u32,
    layout: std.builtin.Type.ContainerLayout,
    any_comptime_fields: bool,
    any_field_defaults: bool,
    any_field_aligns: bool,
    /// Explicitly specified backing int type. `.none` if not packed or if backing type is inferred.
    packed_backing_int_type: Index,
}) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .struct_type = .{ .reified = .{
        .zir_index = ini.zir_index,
        .type_hash = ini.type_hash,
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const field_name_map = try ip.addMap(gpa, io, tid, ini.fields_len);
    errdefer local.mutate.maps.len -= 1;

    const is_extern = switch (ini.layout) {
        .auto => false,
        .@"extern" => true,
        .@"packed" => {
            try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeStructPacked).@"struct".fields.len +
                2 + // type_hash
                ini.fields_len + // field_name
                ini.fields_len + // field_type
                (if (ini.any_field_defaults) ini.fields_len else 0)); // field_default

            const extra_index = addExtraAssumeCapacity(extra, Tag.TypeStructPacked{
                .zir_index = ini.zir_index,
                .bits = .{
                    .captures_len = .reified,
                    .want_layout = false,
                },
                .name = undefined, // set by `finish`
                .name_nav = undefined, // set by `finish`
                .namespace = undefined, // set by `finish`
                .backing_int_type = ini.packed_backing_int_type,
                .fields_len = ini.fields_len,
                .field_name_map = field_name_map,
            });
            _ = addExtraAssumeCapacity(extra, PackedU64.init(ini.type_hash)); // type_hash
            const field_names_start = extra.mutate.len;
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
            const field_types_start = extra.mutate.len;
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
            const field_defaults_start = extra.mutate.len;
            if (ini.any_field_defaults) {
                extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_default
            }
            items.appendAssumeCapacity(.{
                .tag = switch (ini.packed_backing_int_type) {
                    .none => if (ini.any_field_defaults) .type_struct_packed_auto_defaults else .type_struct_packed_auto,
                    else => if (ini.any_field_defaults) .type_struct_packed_explicit_defaults else .type_struct_packed_explicit,
                },
                .data = extra_index,
            });
            return .{ .wip = .{
                .index = gop.put(),
                .tid = tid,
                .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeStructPacked, "name").?,
                .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeStructPacked, "name_nav").?,
                .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeStructPacked, "namespace").?,
                .field_names = .{ .tid = tid, .start = field_names_start, .len = ini.fields_len },
                .field_types = .{ .tid = tid, .start = field_types_start, .len = ini.fields_len },
                .field_values = if (ini.any_field_defaults)
                    .{ .tid = tid, .start = field_defaults_start, .len = ini.fields_len }
                else
                    undefined,
                .field_aligns = undefined,
                .field_is_comptime_bits = undefined,
            } };
        },
    };

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeStruct).@"struct".fields.len +
        2 + // type_hash
        ini.fields_len + // field_name
        ini.fields_len + // field_type
        (if (ini.any_field_defaults) ini.fields_len else 0) + // field_default
        (if (ini.any_field_aligns) (ini.fields_len + 3) / 4 else 0) + // field_align
        (if (ini.any_comptime_fields) (ini.fields_len + 31) / 32 else 0) + // field_is_comptime_bits
        (if (!is_extern) ini.fields_len else 0) + // field_runtime_order
        ini.fields_len); // field_offset

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeStruct{
        .zir_index = ini.zir_index,
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .fields_len = ini.fields_len,
        .field_name_map = field_name_map,
        .size = 0,
        .flags = .{
            .any_captures = .reified,
            .layout = if (is_extern) .@"extern" else .auto,
            .any_comptime_fields = ini.any_comptime_fields,
            .any_field_defaults = ini.any_field_defaults,
            .any_field_aligns = ini.any_field_aligns,
            .class = .no_possible_value,
            .alignment = .none,
            .want_layout = false,
        },
    });
    _ = addExtraAssumeCapacity(extra, PackedU64.init(ini.type_hash)); // type_hash
    const field_names_start = extra.mutate.len;
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
    const field_types_start = extra.mutate.len;
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
    const field_defaults_start = extra.mutate.len;
    if (ini.any_field_defaults) {
        extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_default
    }
    const field_aligns_start = extra.mutate.len;
    if (ini.any_field_aligns) {
        extra.appendNTimesAssumeCapacity(.{0}, (ini.fields_len + 3) / 4); // field_align
    }
    const field_is_comptime_bits_start = extra.mutate.len;
    if (ini.any_comptime_fields) {
        extra.appendNTimesAssumeCapacity(.{0}, (ini.fields_len + 31) / 32); // field_is_comptime_bits
    }
    if (!is_extern) {
        extra.appendNTimesAssumeCapacity(.{@intFromEnum(LoadedStructType.RuntimeOrder.unresolved)}, ini.fields_len); // field_runtime_order
    }
    extra.appendNTimesAssumeCapacity(.{0}, ini.fields_len); // field_offset
    items.appendAssumeCapacity(.{
        .tag = .type_struct,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeStruct, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeStruct, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeStruct, "namespace").?,
        .field_names = .{ .tid = tid, .start = field_names_start, .len = ini.fields_len },
        .field_types = .{ .tid = tid, .start = field_types_start, .len = ini.fields_len },
        .field_values = if (ini.any_field_defaults)
            .{ .tid = tid, .start = field_defaults_start, .len = ini.fields_len }
        else
            undefined,
        .field_aligns = if (ini.any_field_aligns)
            .{ .tid = tid, .start = field_aligns_start, .len = ini.fields_len }
        else
            undefined,
        .field_is_comptime_bits = if (ini.any_comptime_fields)
            .{ .tid = tid, .start = field_is_comptime_bits_start, .len = (ini.fields_len + 31) / 32 }
        else
            undefined,
    } };
}

pub fn getDeclaredUnionType(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    ini: struct {
        zir_index: TrackedInst.Index,
        captures: []const CaptureValue,

        // If the value of any of the following fields would change on an incremental update, then logic
        // in `Zcu.mapOldZirToNew` must detect that (these properties are all trivially known from ZIR)
        // and refuse to map the type declaration. This causes `zir_index` to change so that a new type
        // will be interned at a fresh index.
        //
        // In the future, it would be good to remove all of those fields from `ini`, and in fact just
        // have a single function `getDeclaredContainer` which is suitable for all container types.
        // However, this requires some major changes to how container types are represented in the
        // InternPool, so that it is possible for their backing storage to be "reallocated" as needed
        // during type resolution.
        fields_len: u32,
        layout: std.builtin.Type.ContainerLayout,
        any_field_aligns: bool,
        tag_usage: LoadedUnionType.TagUsage,
        enum_tag_mode: BackingTypeMode,
        packed_backing_mode: BackingTypeMode,
    },
) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .union_type = .{ .declared = .{
        .zir_index = ini.zir_index,
        .captures = .{ .external = ini.captures },
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const is_extern = switch (ini.layout) {
        .auto => false,
        .@"extern" => true,
        .@"packed" => {
            try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeUnionPacked).@"struct".fields.len +
                ini.captures.len + // capture
                ini.fields_len); // field_type

            const extra_index = addExtraAssumeCapacity(extra, Tag.TypeUnionPacked{
                .zir_index = ini.zir_index,
                .bits = .{
                    .captures_len = @enumFromInt(ini.captures.len),
                    .want_layout = false,
                },
                .name = undefined, // set by `finish`
                .name_nav = undefined, // set by `finish`
                .namespace = undefined, // set by `finish`
                .backing_int_type = .none,
                .enum_tag_type = .none,
                .fields_len = ini.fields_len,
            });
            extra.appendSliceAssumeCapacity(.{@ptrCast(ini.captures)}); // capture
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
            items.appendAssumeCapacity(.{
                .tag = switch (ini.packed_backing_mode) {
                    .auto => .type_union_packed_auto,
                    .explicit => .type_union_packed_explicit,
                },
                .data = extra_index,
            });
            return .{ .wip = .{
                .index = gop.put(),
                .tid = tid,
                .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeUnionPacked, "name").?,
                .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeUnionPacked, "name_nav").?,
                .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeUnionPacked, "namespace").?,
                .field_names = undefined,
                .field_types = undefined,
                .field_values = undefined,
                .field_aligns = undefined,
                .field_is_comptime_bits = undefined,
            } };
        },
    };

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeUnion).@"struct".fields.len +
        1 + // captures_len
        ini.captures.len + // capture
        ini.fields_len + // field_type
        (if (ini.any_field_aligns) (ini.fields_len + 3) / 4 else 0)); // field_align

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeUnion{
        .zir_index = ini.zir_index,
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .enum_tag_type = .none,
        .fields_len = ini.fields_len,
        .size = 0,
        .padding = 0,
        .flags = .{
            .any_captures = if (ini.captures.len != 0) .true else .false,
            .enum_tag_mode = ini.enum_tag_mode,
            .layout = if (is_extern) .@"extern" else .auto,
            .any_field_aligns = ini.any_field_aligns,
            .tag_usage = ini.tag_usage,
            .class = .no_possible_value,
            .has_runtime_tag = false,
            .alignment = .none,
            .want_layout = false,
        },
    });
    if (ini.captures.len > 0) {
        extra.appendAssumeCapacity(.{@intCast(ini.captures.len)}); // captures_len
        extra.appendSliceAssumeCapacity(.{@ptrCast(ini.captures)}); // capture
    }
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
    if (ini.any_field_aligns) {
        extra.appendNTimesAssumeCapacity(.{0}, (ini.fields_len + 3) / 4); // field_align
    }
    items.appendAssumeCapacity(.{
        .tag = .type_union,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeUnion, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeUnion, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeUnion, "namespace").?,
        .field_names = undefined,
        .field_types = undefined,
        .field_values = undefined,
        .field_aligns = undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub fn getReifiedUnionType(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, ini: struct {
    zir_index: TrackedInst.Index,
    type_hash: u64,
    fields_len: u32,
    layout: std.builtin.Type.ContainerLayout,
    any_field_aligns: bool,
    tag_usage: LoadedUnionType.TagUsage,
    /// Explicitly specified enum tag type. `.none` if `tag_usage != .tagged`.
    enum_tag_type: Index,
    /// Explicitly specified backing int type. `.none` if not packed or if backing type is inferred.
    packed_backing_int_type: Index,
}) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .union_type = .{ .reified = .{
        .zir_index = ini.zir_index,
        .type_hash = ini.type_hash,
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const is_extern = switch (ini.layout) {
        .auto => false,
        .@"extern" => true,
        .@"packed" => {
            try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeUnionPacked).@"struct".fields.len +
                2 + // type_hash
                ini.fields_len + // reified_field_name
                ini.fields_len); // field_type

            const extra_index = addExtraAssumeCapacity(extra, Tag.TypeUnionPacked{
                .zir_index = ini.zir_index,
                .bits = .{
                    .captures_len = .reified,
                    .want_layout = false,
                },
                .name = undefined, // set by `finish`
                .name_nav = undefined, // set by `finish`
                .namespace = undefined, // set by `finish`
                .backing_int_type = ini.packed_backing_int_type,
                .enum_tag_type = .none,
                .fields_len = ini.fields_len,
            });
            _ = addExtraAssumeCapacity(extra, PackedU64.init(ini.type_hash)); // type_hash
            const field_names_start = extra.mutate.len;
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // reified_field_name
            const field_types_start = extra.mutate.len;
            extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
            items.appendAssumeCapacity(.{
                .tag = switch (ini.packed_backing_int_type) {
                    .none => .type_union_packed_auto,
                    else => .type_union_packed_explicit,
                },
                .data = extra_index,
            });
            return .{ .wip = .{
                .index = gop.put(),
                .tid = tid,
                .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeUnionPacked, "name").?,
                .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeUnionPacked, "name_nav").?,
                .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeUnionPacked, "namespace").?,
                .field_names = .{ .tid = tid, .start = field_names_start, .len = ini.fields_len },
                .field_types = .{ .tid = tid, .start = field_types_start, .len = ini.fields_len },
                .field_values = undefined,
                .field_aligns = undefined,
                .field_is_comptime_bits = undefined,
            } };
        },
    };

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeUnion).@"struct".fields.len +
        2 + // type_hash
        ini.fields_len + // reified_field_name
        ini.fields_len + // field_type
        (if (ini.any_field_aligns) (ini.fields_len + 3) / 4 else 0)); // field_align

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeUnion{
        .zir_index = ini.zir_index,
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .enum_tag_type = ini.enum_tag_type,
        .fields_len = ini.fields_len,
        .size = 0,
        .padding = 0,
        .flags = .{
            .any_captures = .reified,
            .enum_tag_mode = if (ini.enum_tag_type == .none) .auto else .explicit,
            .layout = if (is_extern) .@"extern" else .auto,
            .any_field_aligns = ini.any_field_aligns,
            .tag_usage = ini.tag_usage,
            .class = .no_possible_value,
            .has_runtime_tag = false,
            .alignment = .none,
            .want_layout = false,
        },
    });
    _ = addExtraAssumeCapacity(extra, PackedU64.init(ini.type_hash));
    const field_names_start = extra.mutate.len;
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // reified_field_name
    const field_types_start = extra.mutate.len;
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_type
    const field_aligns_start = extra.mutate.len;
    if (ini.any_field_aligns) {
        extra.appendNTimesAssumeCapacity(.{0}, (ini.fields_len + 3) / 4); // field_align
    }
    items.appendAssumeCapacity(.{
        .tag = .type_union,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeUnion, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeUnion, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeUnion, "namespace").?,
        .field_names = .{ .tid = tid, .start = field_names_start, .len = ini.fields_len },
        .field_types = .{ .tid = tid, .start = field_types_start, .len = ini.fields_len },
        .field_values = undefined,
        .field_aligns = if (ini.any_field_aligns)
            .{ .tid = tid, .start = field_aligns_start, .len = ini.fields_len }
        else
            undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub fn getDeclaredEnumType(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    ini: struct {
        zir_index: TrackedInst.Index,
        captures: []const CaptureValue,

        // If the value of any of the following fields would change on an incremental update, then logic
        // in `Zcu.mapOldZirToNew` must detect that (these properties are all trivially known from ZIR)
        // and refuse to map the type declaration. This causes `zir_index` to change so that a new type
        // will be interned at a fresh index.
        //
        // In the future, it would be good to remove all of those fields from `ini`, and in fact just
        // have a single function `getDeclaredContainer` which is suitable for all container types.
        // However, this requires some major changes to how container types are represented in the
        // InternPool, so that it is possible for their backing storage to be "reallocated" as needed
        // during type resolution.
        fields_len: u32,
        nonexhaustive: bool,
        /// For `enum(T)` this is `.explicit`. Otherwise this is `.none`.
        int_tag_mode: BackingTypeMode,
    },
) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .enum_type = .{ .declared = .{
        .zir_index = ini.zir_index,
        .captures = .{ .external = ini.captures },
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const tag: Tag, const have_values: bool = if (ini.nonexhaustive)
        .{ .type_enum_nonexhaustive, true }
    else if (ini.int_tag_mode == .explicit)
        .{ .type_enum_explicit, true }
    else
        .{ .type_enum_auto, false };

    const field_name_map = try ip.addMap(gpa, io, tid, ini.fields_len);
    errdefer local.mutate.maps.len -= 1;

    const field_value_map = if (have_values) try ip.addMap(gpa, io, tid, ini.fields_len) else undefined;
    errdefer local.mutate.maps.len -= @intFromBool(have_values);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeEnum).@"struct".fields.len +
        1 + // zir_index
        ini.captures.len + // capture
        @intFromBool(have_values) + // field_value_map
        ini.fields_len + // field_name
        (if (have_values) ini.fields_len else 0)); // field_value

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeEnum{
        .bits = .{
            .captures_len = @enumFromInt(ini.captures.len),
            .want_layout = false,
        },
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .int_tag_type = .none,
        .fields_len = ini.fields_len,
        .field_name_map = field_name_map,
    });
    extra.appendAssumeCapacity(.{@intFromEnum(ini.zir_index)}); // zir_index
    extra.appendSliceAssumeCapacity(.{@ptrCast(ini.captures)}); // capture
    if (have_values) extra.appendAssumeCapacity(.{@intFromEnum(field_value_map)}); // field_value_map
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
    if (have_values) extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_value
    items.appendAssumeCapacity(.{
        .tag = tag,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "namespace").?,
        .field_names = undefined,
        .field_types = undefined,
        .field_values = undefined,
        .field_aligns = undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub fn getReifiedEnumType(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, ini: struct {
    zir_index: TrackedInst.Index,
    type_hash: u64,
    fields_len: u32,
    nonexhaustive: bool,
    /// Explicitly specified int tag type, or `.none` if the int tag type is inferred.
    int_tag_type: Index,
}) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .enum_type = .{ .reified = .{
        .zir_index = ini.zir_index,
        .type_hash = ini.type_hash,
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const tag: Tag, const have_values: bool = if (ini.nonexhaustive)
        .{ .type_enum_nonexhaustive, true }
    else if (ini.int_tag_type != .none)
        .{ .type_enum_explicit, true }
    else
        .{ .type_enum_auto, false };

    const field_name_map = try ip.addMap(gpa, io, tid, ini.fields_len);
    errdefer local.mutate.maps.len -= 1;

    const field_value_map = if (have_values) try ip.addMap(gpa, io, tid, ini.fields_len) else undefined;
    errdefer local.mutate.maps.len -= @intFromBool(have_values);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeEnum).@"struct".fields.len +
        1 + // zir_index
        2 + // type_hash
        @intFromBool(have_values) + // field_value_map
        ini.fields_len + // field_name
        (if (have_values) ini.fields_len else 0)); // field_value

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeEnum{
        .bits = .{
            .captures_len = .reified,
            .want_layout = false,
        },
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .int_tag_type = ini.int_tag_type,
        .fields_len = ini.fields_len,
        .field_name_map = field_name_map,
    });
    extra.appendAssumeCapacity(.{@intFromEnum(ini.zir_index)}); // zir_index
    _ = addExtraAssumeCapacity(extra, PackedU64.init(ini.type_hash)); // type_hash
    if (have_values) extra.appendAssumeCapacity(.{@intFromEnum(field_value_map)}); // field_value_map
    const field_names_start = extra.mutate.len;
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
    const field_values_start = extra.mutate.len;
    if (have_values) extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_value
    items.appendAssumeCapacity(.{
        .tag = tag,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "namespace").?,
        .field_names = .{ .tid = tid, .start = field_names_start, .len = ini.fields_len },
        .field_types = undefined,
        .field_values = if (have_values)
            .{ .tid = tid, .start = field_values_start, .len = ini.fields_len }
        else
            undefined,
        .field_aligns = undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub fn getGeneratedEnumTagType(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, ini: struct {
    /// The union type for which this enum is a generated tag.
    union_type: Index,
    /// For `union(enum(T))` this is `.explicit`. Otherwise this is `.none`.
    int_tag_mode: BackingTypeMode,
    fields_len: u32,
}) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .enum_type = .{ .generated_union_tag = ini.union_type } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    const field_name_map = try ip.addMap(gpa, io, tid, ini.fields_len);
    errdefer local.mutate.maps.len -= 1;

    const have_values = switch (ini.int_tag_mode) {
        .explicit => true,
        .auto => false,
    };

    const field_value_map = if (have_values) try ip.addMap(gpa, io, tid, ini.fields_len) else undefined;
    errdefer local.mutate.maps.len -= @intFromBool(have_values);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeEnum).@"struct".fields.len +
        1 + // owner_union
        @intFromBool(have_values) + // field_value_map
        ini.fields_len + // field_name
        (if (have_values) ini.fields_len else 0)); // field_value

    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeEnum{
        .bits = .{
            .captures_len = .generated_union_tag,
            .want_layout = false,
        },
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
        .int_tag_type = .none,
        .fields_len = ini.fields_len,
        .field_name_map = field_name_map,
    });
    extra.appendAssumeCapacity(.{@intFromEnum(ini.union_type)}); // owner_union
    if (have_values) extra.appendAssumeCapacity(.{@intFromEnum(field_value_map)});
    extra.appendNTimesAssumeCapacity(.{@intFromEnum(NullTerminatedString.empty)}, ini.fields_len); // field_name
    if (have_values) extra.appendNTimesAssumeCapacity(.{@intFromEnum(Index.none)}, ini.fields_len); // field_value
    items.appendAssumeCapacity(.{
        .tag = switch (ini.int_tag_mode) {
            .auto => .type_enum_auto,
            .explicit => .type_enum_explicit,
        },
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeEnum, "namespace").?,
        .field_names = undefined,
        .field_types = undefined,
        .field_values = undefined,
        .field_aligns = undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub fn getDeclaredOpaqueType(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, ini: struct {
    zir_index: TrackedInst.Index,
    captures: []const CaptureValue,
}) Allocator.Error!WipContainerType.Result {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .opaque_type = .{ .declared = .{
        .zir_index = ini.zir_index,
        .captures = .{ .external = ini.captures },
    } } });
    defer gop.deinit();
    if (gop == .existing) return .{ .existing = gop.existing };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeOpaque).@"struct".fields.len + ini.captures.len);
    const extra_index = addExtraAssumeCapacity(extra, Tag.TypeOpaque{
        .zir_index = ini.zir_index,
        .captures_len = @intCast(ini.captures.len),
        .name = undefined, // set by `finish`
        .name_nav = undefined, // set by `finish`
        .namespace = undefined, // set by `finish`
    });
    extra.appendSliceAssumeCapacity(.{@ptrCast(ini.captures)});
    items.appendAssumeCapacity(.{
        .tag = .type_opaque,
        .data = extra_index,
    });
    return .{ .wip = .{
        .index = gop.put(),
        .tid = tid,
        .type_name_index = extra_index + std.meta.fieldIndex(Tag.TypeOpaque, "name").?,
        .name_nav_index = extra_index + std.meta.fieldIndex(Tag.TypeOpaque, "name_nav").?,
        .namespace_index = extra_index + std.meta.fieldIndex(Tag.TypeOpaque, "namespace").?,
        .field_names = undefined,
        .field_types = undefined,
        .field_values = undefined,
        .field_aligns = undefined,
        .field_is_comptime_bits = undefined,
    } };
}

pub const WipContainerType = struct {
    index: Index,
    tid: Zcu.PerThread.Id,
    type_name_index: u32,
    name_nav_index: u32,
    namespace_index: u32,

    // These fields are only populated when creating reified types, because reified types populate
    // field information immediately, with type resolution only handling validation. This is in
    // contrast to declared types, where field information is populated by the type resolution
    // process evaluating ZIR expressions.
    field_names: NullTerminatedString.Slice,
    field_types: Index.Slice,
    field_values: Index.Slice,
    field_aligns: Alignment.Slice,
    field_is_comptime_bits: LoadedStructType.ComptimeBits,

    pub fn setName(
        wip: WipContainerType,
        ip: *InternPool,
        type_name: NullTerminatedString,
        /// This should be the `Nav` we are named after if we use the `.parent` name strategy; `.none` otherwise.
        /// This is also `.none` if we use `.parent` because we are the root struct type for a file.
        name_nav: Nav.Index.Optional,
    ) void {
        const extra = ip.getLocalShared(wip.tid).extra.acquire();
        const extra_items = extra.view().items(.@"0");
        extra_items[wip.type_name_index] = @intFromEnum(type_name);
        extra_items[wip.name_nav_index] = @intFromEnum(name_nav);
    }

    pub fn finish(
        wip: WipContainerType,
        ip: *InternPool,
        namespace: NamespaceIndex,
    ) Index {
        const extra = ip.getLocalShared(wip.tid).extra.acquire();
        const extra_items = extra.view().items(.@"0");

        extra_items[wip.namespace_index] = @intFromEnum(namespace);

        return wip.index;
    }

    pub fn cancel(wip: WipContainerType, ip: *InternPool, tid: Zcu.PerThread.Id) void {
        ip.remove(tid, wip.index);
    }

    pub const Result = union(enum) {
        wip: WipContainerType,
        existing: Index,
    };
};

pub fn getUnion(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    un: Key.Union,
) Allocator.Error!Index {
    assert(un.ty != .none);
    assert(un.val != .none);
    assert(ip.loadUnionType(un.ty).layout != .@"packed");

    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .un = un });
    defer gop.deinit();
    if (gop == .existing) return gop.existing;
    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);

    items.appendAssumeCapacity(.{
        .tag = .union_value,
        .data = try addExtra(extra, un),
    });

    return gop.put();
}

pub const TupleTypeInit = struct {
    types: []const Index,
    /// These elements may be `none`, indicating runtime-known.
    values: []const Index,
};

pub fn getTupleType(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    ini: TupleTypeInit,
) Allocator.Error!Index {
    assert(ini.types.len == ini.values.len);
    for (ini.types) |elem| assert(elem != .none);

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);

    const prev_extra_len = extra.mutate.len;
    const fields_len: u32 = @intCast(ini.types.len);

    try items.ensureUnusedCapacity(1);
    try extra.ensureUnusedCapacity(
        @typeInfo(TypeTuple).@"struct".fields.len + (fields_len * 3),
    );

    const extra_index = addExtraAssumeCapacity(extra, TypeTuple{
        .fields_len = fields_len,
    });
    extra.appendSliceAssumeCapacity(.{@ptrCast(ini.types)});
    extra.appendSliceAssumeCapacity(.{@ptrCast(ini.values)});
    errdefer extra.mutate.len = prev_extra_len;

    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .tuple_type = extraTypeTuple(tid, extra.list.*, extra_index) });
    defer gop.deinit();
    if (gop == .existing) {
        extra.mutate.len = prev_extra_len;
        return gop.existing;
    }

    items.appendAssumeCapacity(.{
        .tag = .type_tuple,
        .data = extra_index,
    });
    return gop.put();
}

/// This is equivalent to `Key.FuncType` but adjusted to have a slice for `param_types`.
pub const GetFuncTypeKey = struct {
    param_types: []const Index,
    return_type: Index,
    comptime_bits: u32 = 0,
    noalias_bits: u32 = 0,
    /// `null` means generic.
    cc: ?std.builtin.CallingConvention = .auto,
    is_var_args: bool = false,
    is_noinline: bool = false,
};

pub fn getFuncType(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    key: GetFuncTypeKey,
) Allocator.Error!Index {
    // Validate input parameters.
    assert(key.return_type != .none);
    for (key.param_types) |param_type| assert(param_type != .none);

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    try items.ensureUnusedCapacity(1);
    const extra = local.getMutableExtra(gpa, io);

    // The strategy here is to add the function type unconditionally, then to
    // ask if it already exists, and if so, revert the lengths of the mutated
    // arrays. This is similar to what `getOrPutTrailingString` does.
    const prev_extra_len = extra.mutate.len;
    const params_len: u32 = @intCast(key.param_types.len);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.TypeFunction).@"struct".fields.len +
        @intFromBool(key.comptime_bits != 0) +
        @intFromBool(key.noalias_bits != 0) +
        params_len);

    const func_type_extra_index = addExtraAssumeCapacity(extra, Tag.TypeFunction{
        .params_len = params_len,
        .return_type = key.return_type,
        .flags = .{
            .cc = .pack(key.cc orelse .auto),
            .is_var_args = key.is_var_args,
            .has_comptime_bits = key.comptime_bits != 0,
            .has_noalias_bits = key.noalias_bits != 0,
            .is_noinline = key.is_noinline,
        },
    });

    if (key.comptime_bits != 0) extra.appendAssumeCapacity(.{key.comptime_bits});
    if (key.noalias_bits != 0) extra.appendAssumeCapacity(.{key.noalias_bits});
    extra.appendSliceAssumeCapacity(.{@ptrCast(key.param_types)});
    errdefer extra.mutate.len = prev_extra_len;

    var gop = try ip.getOrPutKey(gpa, io, tid, .{
        .func_type = extraFuncType(tid, extra.list.*, func_type_extra_index),
    });
    defer gop.deinit();
    if (gop == .existing) {
        extra.mutate.len = prev_extra_len;
        return gop.existing;
    }

    items.appendAssumeCapacity(.{
        .tag = .type_function,
        .data = func_type_extra_index,
    });
    return gop.put();
}

/// Intern an `.@"extern"`, creating a corresponding owner `Nav` if necessary.
/// This will *not* queue the extern for codegen: see `Zcu.PerThread.getExtern` for a wrapper which does.
pub fn getExtern(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    /// `key.owner_nav` is ignored.
    key: Key.Extern,
) Allocator.Error!struct {
    index: Index,
    /// Only set if the `Nav` was newly created.
    new_nav: Nav.Index.Optional,
} {
    var gop = try ip.getOrPutKey(gpa, io, tid, .{ .@"extern" = key });
    defer gop.deinit();
    if (gop == .existing) return .{
        .index = gop.existing,
        .new_nav = .none,
    };

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(1);
    try extra.ensureUnusedCapacity(@typeInfo(Tag.Extern).@"struct".fields.len);
    try local.getMutableNavs(gpa, io).ensureUnusedCapacity(1);

    // Predict the index the `@"extern" will live at, so we can construct the owner `Nav` before releasing the shard's mutex.
    const extern_index = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len,
    }, ip);
    const owner_nav = ip.createNav(gpa, io, tid, .{
        .name = key.name,
        .fqn = key.name,
        .val = extern_index,
        .is_const = key.is_const,
        .alignment = key.alignment,
        .@"linksection" = .none,
        .@"addrspace" = key.@"addrspace",
    }) catch unreachable; // capacity asserted above
    const decoration_type, const location_or_descriptor_set, const descriptor_binding = if (key.decoration) |decoration| switch (decoration) {
        .location => |location| .{ Tag.Extern.Flags.DecorationType.location, location, undefined },
        .descriptor => |descriptor| .{ Tag.Extern.Flags.DecorationType.descriptor, descriptor.binding, descriptor.set },
    } else .{ Tag.Extern.Flags.DecorationType.none, undefined, undefined };
    const extra_index = addExtraAssumeCapacity(extra, Tag.Extern{
        .ty = key.ty,
        .lib_name = key.lib_name,
        .location_or_descriptor_set = location_or_descriptor_set,
        .descriptor_binding = descriptor_binding,
        .flags = .{
            .linkage = key.linkage,
            .visibility = key.visibility,
            .is_threadlocal = key.is_threadlocal,
            .is_dll_import = key.is_dll_import,
            .relocation = key.relocation,
            .decoration_type = decoration_type,
            .source = key.source,
        },
        .zir_index = key.zir_index,
        .owner_nav = owner_nav,
    });
    items.appendAssumeCapacity(.{
        .tag = .@"extern",
        .data = extra_index,
    });
    assert(gop.put() == extern_index);

    return .{
        .index = extern_index,
        .new_nav = owner_nav.toOptional(),
    };
}

pub const GetFuncDeclKey = struct {
    owner_nav: Nav.Index,
    ty: Index,
    zir_body_inst: TrackedInst.Index,
    lbrace_line: u32,
    rbrace_line: u32,
    lbrace_column: u32,
    rbrace_column: u32,
    cc: ?std.builtin.CallingConvention,
    is_noinline: bool,
};

pub fn getFuncDecl(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    key: GetFuncDeclKey,
) Allocator.Error!Index {
    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    try items.ensureUnusedCapacity(1);
    const extra = local.getMutableExtra(gpa, io);

    // The strategy here is to add the function type unconditionally, then to
    // ask if it already exists, and if so, revert the lengths of the mutated
    // arrays. This is similar to what `getOrPutTrailingString` does.
    const prev_extra_len = extra.mutate.len;

    try extra.ensureUnusedCapacity(@typeInfo(Tag.FuncDecl).@"struct".fields.len);

    const func_decl_extra_index = addExtraAssumeCapacity(extra, Tag.FuncDecl{
        .analysis = .{
            .want_runtime_analysis = false,
            .branch_hint = .none,
            .is_noinline = key.is_noinline,
            .has_error_trace = false,
            .inferred_error_set = false,
            .disable_instrumentation = false,
            .disable_intrinsics = false,
        },
        .owner_nav = key.owner_nav,
        .ty = key.ty,
        .zir_body_inst = key.zir_body_inst,
        .lbrace_line = key.lbrace_line,
        .rbrace_line = key.rbrace_line,
        .lbrace_column = key.lbrace_column,
        .rbrace_column = key.rbrace_column,
    });
    errdefer extra.mutate.len = prev_extra_len;

    var gop = try ip.getOrPutKey(gpa, io, tid, .{
        .func = extraFuncDecl(tid, extra.list.*, func_decl_extra_index),
    });
    defer gop.deinit();
    if (gop == .existing) {
        extra.mutate.len = prev_extra_len;

        const zir_body_inst_ptr = ip.funcDeclInfo(gop.existing).zirBodyInstPtr(ip);
        if (zir_body_inst_ptr.* != key.zir_body_inst) {
            // Since this function's `owner_nav` matches `key`, this *is* the function we're talking
            // about. The only way it could have a different ZIR `func` instruction is if the old
            // instruction has been lost and replaced with a new `TrackedInst.Index`.
            assert(zir_body_inst_ptr.resolve(ip) == null);
            zir_body_inst_ptr.* = key.zir_body_inst;
        }

        return gop.existing;
    }

    items.appendAssumeCapacity(.{
        .tag = .func_decl,
        .data = func_decl_extra_index,
    });
    return gop.put();
}

pub const GetFuncDeclIesKey = struct {
    owner_nav: Nav.Index,
    param_types: []Index,
    noalias_bits: u32,
    comptime_bits: u32,
    bare_return_type: Index,
    /// null means generic.
    cc: ?std.builtin.CallingConvention,
    is_var_args: bool,
    is_noinline: bool,
    zir_body_inst: TrackedInst.Index,
    lbrace_line: u32,
    rbrace_line: u32,
    lbrace_column: u32,
    rbrace_column: u32,
};

pub fn getFuncDeclIes(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    key: GetFuncDeclIesKey,
) Allocator.Error!Index {
    // Validate input parameters.
    assert(key.bare_return_type != .none);
    for (key.param_types) |param_type| assert(param_type != .none);

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    try items.ensureUnusedCapacity(4);
    const extra = local.getMutableExtra(gpa, io);

    // The strategy here is to add the function decl unconditionally, then to
    // ask if it already exists, and if so, revert the lengths of the mutated
    // arrays. This is similar to what `getOrPutTrailingString` does.
    const prev_extra_len = extra.mutate.len;
    const params_len: u32 = @intCast(key.param_types.len);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.FuncDecl).@"struct".fields.len +
        1 + // inferred_error_set
        @typeInfo(Tag.ErrorUnionType).@"struct".fields.len +
        @typeInfo(Tag.TypeFunction).@"struct".fields.len +
        @intFromBool(key.comptime_bits != 0) +
        @intFromBool(key.noalias_bits != 0) +
        params_len);

    const func_index = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 0,
    }, ip);
    const error_union_type = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 1,
    }, ip);
    const error_set_type = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 2,
    }, ip);
    const func_ty = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 3,
    }, ip);

    const func_decl_extra_index = addExtraAssumeCapacity(extra, Tag.FuncDecl{
        .analysis = .{
            .want_runtime_analysis = false,
            .branch_hint = .none,
            .is_noinline = key.is_noinline,
            .has_error_trace = false,
            .inferred_error_set = true,
            .disable_instrumentation = false,
            .disable_intrinsics = false,
        },
        .owner_nav = key.owner_nav,
        .ty = func_ty,
        .zir_body_inst = key.zir_body_inst,
        .lbrace_line = key.lbrace_line,
        .rbrace_line = key.rbrace_line,
        .lbrace_column = key.lbrace_column,
        .rbrace_column = key.rbrace_column,
    });
    extra.appendAssumeCapacity(.{@intFromEnum(Index.none)});

    const func_type_extra_index = addExtraAssumeCapacity(extra, Tag.TypeFunction{
        .params_len = params_len,
        .return_type = error_union_type,
        .flags = .{
            .cc = .pack(key.cc orelse .auto),
            .is_var_args = key.is_var_args,
            .has_comptime_bits = key.comptime_bits != 0,
            .has_noalias_bits = key.noalias_bits != 0,
            .is_noinline = key.is_noinline,
        },
    });
    if (key.comptime_bits != 0) extra.appendAssumeCapacity(.{key.comptime_bits});
    if (key.noalias_bits != 0) extra.appendAssumeCapacity(.{key.noalias_bits});
    extra.appendSliceAssumeCapacity(.{@ptrCast(key.param_types)});

    items.appendSliceAssumeCapacity(.{
        .tag = &.{
            .func_decl,
            .type_error_union,
            .type_inferred_error_set,
            .type_function,
        },
        .data = &.{
            func_decl_extra_index,
            addExtraAssumeCapacity(extra, Tag.ErrorUnionType{
                .error_set_type = error_set_type,
                .payload_type = key.bare_return_type,
            }),
            @intFromEnum(func_index),
            func_type_extra_index,
        },
    });
    errdefer {
        items.mutate.len -= 4;
        extra.mutate.len = prev_extra_len;
    }

    var func_gop = try ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, .{
        .func = extraFuncDecl(tid, extra.list.*, func_decl_extra_index),
    }, 3);
    defer func_gop.deinit();
    if (func_gop == .existing) {
        // An existing function type was found; undo the additions to our two arrays.
        items.mutate.len -= 4;
        extra.mutate.len = prev_extra_len;

        const zir_body_inst_ptr = ip.funcDeclInfo(func_gop.existing).zirBodyInstPtr(ip);
        if (zir_body_inst_ptr.* != key.zir_body_inst) {
            // Since this function's `owner_nav` matches `key`, this *is* the function we're talking
            // about. The only way it could have a different ZIR `func` instruction is if the old
            // instruction has been lost and replaced with a new `TrackedInst.Index`.
            assert(zir_body_inst_ptr.resolve(ip) == null);
            zir_body_inst_ptr.* = key.zir_body_inst;
        }

        return func_gop.existing;
    }
    func_gop.putTentative(func_index);
    var error_union_type_gop = try ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, .{ .error_union_type = .{
        .error_set_type = error_set_type,
        .payload_type = key.bare_return_type,
    } }, 2);
    defer error_union_type_gop.deinit();
    error_union_type_gop.putTentative(error_union_type);
    var error_set_type_gop = try ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, .{
        .inferred_error_set_type = func_index,
    }, 1);
    defer error_set_type_gop.deinit();
    error_set_type_gop.putTentative(error_set_type);
    var func_ty_gop = try ip.getOrPutKey(gpa, io, tid, .{
        .func_type = extraFuncType(tid, extra.list.*, func_type_extra_index),
    });
    defer func_ty_gop.deinit();
    func_ty_gop.putTentative(func_ty);

    func_gop.putFinal(func_index);
    error_union_type_gop.putFinal(error_union_type);
    error_set_type_gop.putFinal(error_set_type);
    func_ty_gop.putFinal(func_ty);
    return func_index;
}

pub fn getErrorSetType(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    names: []const NullTerminatedString,
) Allocator.Error!Index {
    assert(std.sort.isSorted(NullTerminatedString, names, {}, NullTerminatedString.indexLessThan));

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try extra.ensureUnusedCapacity(@typeInfo(Tag.ErrorSet).@"struct".fields.len + names.len);

    const names_map = try ip.addMap(gpa, io, tid, names.len);
    errdefer local.mutate.maps.len -= 1;

    // The strategy here is to add the type unconditionally, then to ask if it
    // already exists, and if so, revert the lengths of the mutated arrays.
    // This is similar to what `getOrPutTrailingString` does.
    const prev_extra_len = extra.mutate.len;
    errdefer extra.mutate.len = prev_extra_len;

    const error_set_extra_index = addExtraAssumeCapacity(extra, Tag.ErrorSet{
        .names_len = @intCast(names.len),
        .names_map = names_map,
    });
    extra.appendSliceAssumeCapacity(.{@ptrCast(names)});
    errdefer extra.mutate.len = prev_extra_len;

    var gop = try ip.getOrPutKey(gpa, io, tid, .{
        .error_set_type = extraErrorSet(tid, extra.list.*, error_set_extra_index),
    });
    defer gop.deinit();
    if (gop == .existing) {
        extra.mutate.len = prev_extra_len;
        return gop.existing;
    }

    try items.append(.{
        .tag = .type_error_set,
        .data = error_set_extra_index,
    });
    errdefer items.mutate.len -= 1;

    ip.addStringsToMap(names_map, names);

    return gop.put();
}

pub const GetFuncInstanceKey = struct {
    /// Has the length of the instance function (may be lesser than
    /// comptime_args).
    param_types: []Index,
    /// Has the length of generic_owner's parameters (may be greater than
    /// param_types).
    comptime_args: []const Index,
    noalias_bits: u32,
    bare_return_type: Index,
    is_noinline: bool,
    generic_owner: Index,
    inferred_error_set: bool,
};

pub fn getFuncInstance(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    arg: GetFuncInstanceKey,
) Allocator.Error!Index {
    if (arg.inferred_error_set)
        return getFuncInstanceIes(ip, gpa, io, tid, arg);

    const generic_owner = unwrapCoercedFunc(ip, arg.generic_owner);
    const generic_owner_ty = ip.indexToKey(ip.funcDeclInfo(generic_owner).ty).func_type;

    const func_ty = try ip.getFuncType(gpa, io, tid, .{
        .param_types = arg.param_types,
        .return_type = arg.bare_return_type,
        .noalias_bits = arg.noalias_bits,
        .cc = generic_owner_ty.cc,
        .is_noinline = arg.is_noinline,
    });

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try extra.ensureUnusedCapacity(@typeInfo(Tag.FuncInstance).@"struct".fields.len +
        arg.comptime_args.len);

    assert(arg.comptime_args.len == ip.funcTypeParamsLen(ip.typeOf(generic_owner)));

    const prev_extra_len = extra.mutate.len;
    errdefer extra.mutate.len = prev_extra_len;

    const func_extra_index = addExtraAssumeCapacity(extra, Tag.FuncInstance{
        .analysis = .{
            .want_runtime_analysis = false,
            .branch_hint = .none,
            .is_noinline = arg.is_noinline,
            .has_error_trace = false,
            .inferred_error_set = false,
            .disable_instrumentation = false,
            .disable_intrinsics = false,
        },
        // This is populated after we create the Nav below. It is not read
        // by equality or hashing functions.
        .owner_nav = undefined,
        .ty = func_ty,
        .branch_quota = 0,
        .generic_owner = generic_owner,
    });
    extra.appendSliceAssumeCapacity(.{@ptrCast(arg.comptime_args)});

    var gop = try ip.getOrPutKey(gpa, io, tid, .{
        .func = ip.extraFuncInstance(tid, extra.list.*, func_extra_index),
    });
    defer gop.deinit();
    if (gop == .existing) {
        extra.mutate.len = prev_extra_len;
        return gop.existing;
    }

    const func_index = Index.Unwrapped.wrap(.{ .tid = tid, .index = items.mutate.len }, ip);
    try items.append(.{
        .tag = .func_instance,
        .data = func_extra_index,
    });
    errdefer items.mutate.len -= 1;
    try finishFuncInstance(
        ip,
        gpa,
        io,
        tid,
        extra,
        generic_owner,
        func_index,
        func_extra_index,
    );
    return gop.put();
}

/// This function exists separately than `getFuncInstance` because it needs to
/// create 4 new items in the InternPool atomically before it can look for an
/// existing item in the map.
fn getFuncInstanceIes(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    arg: GetFuncInstanceKey,
) Allocator.Error!Index {
    // Validate input parameters.
    assert(arg.inferred_error_set);
    assert(arg.bare_return_type != .none);
    for (arg.param_types) |param_type| assert(param_type != .none);

    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    const extra = local.getMutableExtra(gpa, io);
    try items.ensureUnusedCapacity(4);

    const generic_owner = unwrapCoercedFunc(ip, arg.generic_owner);
    const generic_owner_ty = ip.indexToKey(ip.funcDeclInfo(generic_owner).ty).func_type;

    // The strategy here is to add the function decl unconditionally, then to
    // ask if it already exists, and if so, revert the lengths of the mutated
    // arrays. This is similar to what `getOrPutTrailingString` does.
    const prev_extra_len = extra.mutate.len;
    const params_len: u32 = @intCast(arg.param_types.len);

    try extra.ensureUnusedCapacity(@typeInfo(Tag.FuncInstance).@"struct".fields.len +
        1 + // inferred_error_set
        arg.comptime_args.len +
        @typeInfo(Tag.ErrorUnionType).@"struct".fields.len +
        @typeInfo(Tag.TypeFunction).@"struct".fields.len +
        @intFromBool(arg.noalias_bits != 0) +
        params_len);

    const func_index = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 0,
    }, ip);
    const error_union_type = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 1,
    }, ip);
    const error_set_type = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 2,
    }, ip);
    const func_ty = Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = items.mutate.len + 3,
    }, ip);

    const func_extra_index = addExtraAssumeCapacity(extra, Tag.FuncInstance{
        .analysis = .{
            .want_runtime_analysis = false,
            .branch_hint = .none,
            .is_noinline = arg.is_noinline,
            .has_error_trace = false,
            .inferred_error_set = true,
            .disable_instrumentation = false,
            .disable_intrinsics = false,
        },
        // This is populated after we create the Nav below. It is not read
        // by equality or hashing functions.
        .owner_nav = undefined,
        .ty = func_ty,
        .branch_quota = 0,
        .generic_owner = generic_owner,
    });
    extra.appendAssumeCapacity(.{@intFromEnum(Index.none)}); // resolved error set
    extra.appendSliceAssumeCapacity(.{@ptrCast(arg.comptime_args)});

    const func_type_extra_index = addExtraAssumeCapacity(extra, Tag.TypeFunction{
        .params_len = params_len,
        .return_type = error_union_type,
        .flags = .{
            .cc = .pack(generic_owner_ty.cc),
            .is_var_args = false,
            .has_comptime_bits = false,
            .has_noalias_bits = arg.noalias_bits != 0,
            .is_noinline = arg.is_noinline,
        },
    });
    // no comptime_bits because has_comptime_bits is false
    if (arg.noalias_bits != 0) extra.appendAssumeCapacity(.{arg.noalias_bits});
    extra.appendSliceAssumeCapacity(.{@ptrCast(arg.param_types)});

    items.appendSliceAssumeCapacity(.{
        .tag = &.{
            .func_instance,
            .type_error_union,
            .type_inferred_error_set,
            .type_function,
        },
        .data = &.{
            func_extra_index,
            addExtraAssumeCapacity(extra, Tag.ErrorUnionType{
                .error_set_type = error_set_type,
                .payload_type = arg.bare_return_type,
            }),
            @intFromEnum(func_index),
            func_type_extra_index,
        },
    });
    errdefer {
        items.mutate.len -= 4;
        extra.mutate.len = prev_extra_len;
    }

    var func_gop = try ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, .{
        .func = ip.extraFuncInstance(tid, extra.list.*, func_extra_index),
    }, 3);
    defer func_gop.deinit();
    if (func_gop == .existing) {
        // Hot path: undo the additions to our two arrays.
        items.mutate.len -= 4;
        extra.mutate.len = prev_extra_len;
        return func_gop.existing;
    }
    func_gop.putTentative(func_index);
    var error_union_type_gop = try ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, .{ .error_union_type = .{
        .error_set_type = error_set_type,
        .payload_type = arg.bare_return_type,
    } }, 2);
    defer error_union_type_gop.deinit();
    error_union_type_gop.putTentative(error_union_type);
    var error_set_type_gop = try ip.getOrPutKeyEnsuringAdditionalCapacity(gpa, io, tid, .{
        .inferred_error_set_type = func_index,
    }, 1);
    defer error_set_type_gop.deinit();
    error_set_type_gop.putTentative(error_set_type);
    var func_ty_gop = try ip.getOrPutKey(gpa, io, tid, .{
        .func_type = extraFuncType(tid, extra.list.*, func_type_extra_index),
    });
    defer func_ty_gop.deinit();
    func_ty_gop.putTentative(func_ty);
    try finishFuncInstance(
        ip,
        gpa,
        io,
        tid,
        extra,
        generic_owner,
        func_index,
        func_extra_index,
    );

    func_gop.putFinal(func_index);
    error_union_type_gop.putFinal(error_union_type);
    error_set_type_gop.putFinal(error_set_type);
    func_ty_gop.putFinal(func_ty);
    return func_index;
}

fn finishFuncInstance(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    extra: Local.Extra.Mutable,
    generic_owner: Index,
    func_index: Index,
    func_extra_index: u32,
) Allocator.Error!void {
    const fn_owner_nav = ip.getNav(ip.funcDeclInfo(generic_owner).owner_nav);
    const fn_namespace = fn_owner_nav.analysis.?.namespace;

    // TODO: improve this name
    const nav_name = try ip.getOrPutStringFmt(gpa, io, tid, "{f}__anon_{d}", .{
        fn_owner_nav.name.fmt(ip), @intFromEnum(func_index),
    }, .no_embedded_nulls);
    const nav_index = try ip.createNav(gpa, io, tid, .{
        .name = nav_name,
        .fqn = try ip.namespacePtr(fn_namespace).internFullyQualifiedName(ip, gpa, io, tid, nav_name),
        .val = func_index,
        .is_const = fn_owner_nav.status.fully_resolved.is_const,
        .alignment = fn_owner_nav.status.fully_resolved.alignment,
        .@"linksection" = fn_owner_nav.status.fully_resolved.@"linksection",
        .@"addrspace" = fn_owner_nav.status.fully_resolved.@"addrspace",
    });

    // Populate the owner_nav field which was left undefined until now.
    extra.view().items(.@"0")[
        func_extra_index + std.meta.fieldIndex(Tag.FuncInstance, "owner_nav").?
    ] = @intFromEnum(nav_index);
}

pub fn getIfExists(ip: *const InternPool, key: Key) ?Index {
    const full_hash = key.hash64(ip);
    const hash: u32 = @truncate(full_hash >> 32);
    const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
    const map = shard.shared.map.acquire();
    const map_mask = map.header().mask();
    var map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.acquire();
        if (index == .none) return null;
        if (entry.hash != hash) continue;
        if (ip.indexToKey(index).eql(key, ip)) return index;
    }
}

fn addStringsToMap(
    ip: *InternPool,
    map_index: MapIndex,
    strings: []const NullTerminatedString,
) void {
    const map = map_index.get(ip);
    const adapter: NullTerminatedString.Adapter = .{ .strings = strings };
    for (strings) |string| {
        const gop = map.getOrPutAssumeCapacityAdapted(string, adapter);
        assert(!gop.found_existing);
    }
}

fn addMap(ip: *InternPool, gpa: Allocator, io: Io, tid: Zcu.PerThread.Id, cap: usize) Allocator.Error!MapIndex {
    const maps = ip.getLocal(tid).getMutableMaps(gpa, io);
    const unwrapped: MapIndex.Unwrapped = .{ .tid = tid, .index = maps.mutate.len };
    const ptr = try maps.addOne();
    errdefer maps.mutate.len = unwrapped.index;
    ptr[0].* = .{};
    try ptr[0].ensureTotalCapacity(gpa, cap);
    return unwrapped.wrap(ip);
}

/// This operation only happens under compile error conditions.
/// Leak the index until the next garbage collection.
/// Invalidates all references to this index.
pub fn remove(ip: *InternPool, tid: Zcu.PerThread.Id, index: Index) void {
    const unwrapped_index = index.unwrap(ip);

    if (unwrapped_index.tid == tid) {
        const items_len = &ip.getLocal(unwrapped_index.tid).mutate.items.len;
        if (unwrapped_index.index == items_len.* - 1) {
            // Happy case - we can just drop the item without affecting any other indices.
            items_len.* -= 1;
            return;
        }
    }

    // We must preserve the item so that indices following it remain valid.
    // Thus, we will rewrite the tag to `removed`, leaking the item until
    // next GC but causing `KeyAdapter` to ignore it.
    const items = ip.getLocalShared(unwrapped_index.tid).items.acquire().view();
    @atomicStore(Tag, &items.items(.tag)[unwrapped_index.index], .removed, .unordered);
}

fn addInt(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    ty: Index,
    tag: Tag,
    limbs: []const Limb,
) !void {
    const local = ip.getLocal(tid);
    const items_list = local.getMutableItems(gpa, io);
    const limbs_list = local.getMutableLimbs(gpa, io);
    const limbs_len: u32 = @intCast(limbs.len);
    try limbs_list.ensureUnusedCapacity(Int.limbs_items_len + limbs_len);
    items_list.appendAssumeCapacity(.{
        .tag = tag,
        .data = limbs_list.mutate.len,
    });
    limbs_list.addManyAsArrayAssumeCapacity(Int.limbs_items_len)[0].* = @bitCast(Int{
        .ty = ty,
        .limbs_len = limbs_len,
    });
    limbs_list.appendSliceAssumeCapacity(.{limbs});
}

fn addExtra(extra: Local.Extra.Mutable, item: anytype) Allocator.Error!u32 {
    const fields = @typeInfo(@TypeOf(item)).@"struct".fields;
    try extra.ensureUnusedCapacity(fields.len);
    return addExtraAssumeCapacity(extra, item);
}

fn addExtraAssumeCapacity(extra: Local.Extra.Mutable, item: anytype) u32 {
    const result: u32 = extra.mutate.len;
    inline for (@typeInfo(@TypeOf(item)).@"struct".fields) |field| {
        extra.appendAssumeCapacity(.{switch (field.type) {
            Index,
            Nav.Index,
            Nav.Index.Optional,
            NamespaceIndex,
            OptionalNamespaceIndex,
            MapIndex,
            OptionalMapIndex,
            String,
            NullTerminatedString,
            OptionalNullTerminatedString,
            Tag.TypePointer.VectorIndex,
            TrackedInst.Index,
            TrackedInst.Index.Optional,
            ComptimeAllocIndex,
            => @intFromEnum(@field(item, field.name)),

            u32,
            i32,
            FuncAnalysis,
            Tag.Extern.Flags,
            Tag.TypePointer.Flags,
            Tag.TypeFunction.Flags,
            Tag.TypePointer.PackedOffset,
            Tag.TypeUnion.Flags,
            Tag.TypeStruct.Flags,
            Tag.TypeStructPacked.Bits,
            Tag.TypeUnionPacked.Bits,
            Tag.TypeEnum.Bits,
            => @bitCast(@field(item, field.name)),

            else => @compileError("bad field type: " ++ @typeName(field.type)),
        }});
    }
    return result;
}

fn addLimbsExtraAssumeCapacity(ip: *InternPool, extra: anytype) u32 {
    switch (@sizeOf(Limb)) {
        @sizeOf(u32) => return addExtraAssumeCapacity(ip, extra),
        @sizeOf(u64) => {},
        else => @compileError("unsupported host"),
    }
    const result: u32 = @intCast(ip.limbs.items.len);
    inline for (@typeInfo(@TypeOf(extra)).@"struct".fields, 0..) |field, i| {
        const new: u32 = switch (field.type) {
            u32 => @field(extra, field.name),
            Index => @intFromEnum(@field(extra, field.name)),
            else => @compileError("bad field type: " ++ @typeName(field.type)),
        };
        if (i % 2 == 0) {
            ip.limbs.appendAssumeCapacity(new);
        } else {
            ip.limbs.items[ip.limbs.items.len - 1] |= @as(u64, new) << 32;
        }
    }
    return result;
}

fn extraDataTrail(extra: Local.Extra, comptime T: type, index: u32) struct { data: T, end: u32 } {
    const extra_items = extra.view().items(.@"0");
    var result: T = undefined;
    const fields = @typeInfo(T).@"struct".fields;
    inline for (fields, index..) |field, extra_index| {
        const extra_item = extra_items[extra_index];
        @field(result, field.name) = switch (field.type) {
            Index,
            Nav.Index,
            Nav.Index.Optional,
            NamespaceIndex,
            OptionalNamespaceIndex,
            MapIndex,
            OptionalMapIndex,
            String,
            NullTerminatedString,
            OptionalNullTerminatedString,
            Tag.TypePointer.VectorIndex,
            TrackedInst.Index,
            TrackedInst.Index.Optional,
            ComptimeAllocIndex,
            => @enumFromInt(extra_item),

            u32,
            i32,
            Tag.Extern.Flags,
            Tag.TypePointer.Flags,
            Tag.TypeFunction.Flags,
            Tag.TypePointer.PackedOffset,
            Tag.TypeUnion.Flags,
            Tag.TypeStruct.Flags,
            FuncAnalysis,
            Tag.TypeStructPacked.Bits,
            Tag.TypeUnionPacked.Bits,
            Tag.TypeEnum.Bits,
            => @bitCast(extra_item),

            else => @compileError("bad field type: " ++ @typeName(field.type)),
        };
    }
    return .{
        .data = result,
        .end = @intCast(index + fields.len),
    };
}

fn extraData(extra: Local.Extra, comptime T: type, index: u32) T {
    return extraDataTrail(extra, T, index).data;
}

test "basic usage" {
    const gpa = std.testing.allocator;
    const io = std.testing.io;

    var ip: InternPool = .empty;
    try ip.init(gpa, io, 1);
    defer ip.deinit(gpa, io);

    const i32_type = try ip.get(gpa, io, .main, .{ .int_type = .{
        .signedness = .signed,
        .bits = 32,
    } });
    const array_i32 = try ip.get(gpa, io, .main, .{ .array_type = .{
        .len = 10,
        .child = i32_type,
        .sentinel = .none,
    } });

    const another_i32_type = try ip.get(gpa, io, .main, .{ .int_type = .{
        .signedness = .signed,
        .bits = 32,
    } });
    try std.testing.expect(another_i32_type == i32_type);

    const another_array_i32 = try ip.get(gpa, io, .main, .{ .array_type = .{
        .len = 10,
        .child = i32_type,
        .sentinel = .none,
    } });
    try std.testing.expect(another_array_i32 == array_i32);
}

pub fn childType(ip: *const InternPool, i: Index) Index {
    return switch (ip.indexToKey(i)) {
        .ptr_type => |ptr_type| ptr_type.child,
        .vector_type => |vector_type| vector_type.child,
        .array_type => |array_type| array_type.child,
        .opt_type, .anyframe_type => |child| child,
        else => unreachable,
    };
}

/// Given a slice type, returns the type of the ptr field.
pub fn slicePtrType(ip: *const InternPool, index: Index) Index {
    switch (index) {
        .slice_const_u8_type => return .manyptr_const_u8_type,
        .slice_const_u8_sentinel_0_type => return .manyptr_const_u8_sentinel_0_type,
        .slice_const_slice_const_u8_type => return .manyptr_const_slice_const_u8_type,
        .slice_const_type_type => return .manyptr_const_type_type,
        else => {},
    }
    const item = index.unwrap(ip).getItem(ip);
    switch (item.tag) {
        .type_slice => return @enumFromInt(item.data),
        else => unreachable, // not a slice type
    }
}

/// Given a slice value, returns the value of the ptr field.
pub fn slicePtr(ip: *const InternPool, index: Index) Index {
    const unwrapped_index = index.unwrap(ip);
    const item = unwrapped_index.getItem(ip);
    switch (item.tag) {
        .ptr_slice => return extraData(unwrapped_index.getExtra(ip), PtrSlice, item.data).ptr,
        else => unreachable, // not a slice value
    }
}

/// Given a slice value, returns the value of the len field.
pub fn sliceLen(ip: *const InternPool, index: Index) Index {
    const unwrapped_index = index.unwrap(ip);
    const item = unwrapped_index.getItem(ip);
    switch (item.tag) {
        .ptr_slice => return extraData(unwrapped_index.getExtra(ip), PtrSlice, item.data).len,
        else => unreachable, // not a slice value
    }
}

/// Given an existing value, returns the same value but with the supplied type.
/// Only some combinations are allowed:
/// * identity coercion
/// * undef => any
/// * int <=> int
/// * int <=> enum
/// * enum_literal => enum
/// * float <=> float
/// * ptr <=> ptr
/// * opt ptr <=> ptr
/// * opt ptr <=> opt ptr
/// * int <=> ptr
/// * null_value => opt
/// * payload => opt
/// * error set <=> error set
/// * error union <=> error union
/// * error set => error union
/// * payload => error union
/// * fn <=> fn
/// * aggregate <=> aggregate (where children can also be coerced)
pub fn getCoerced(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    val: Index,
    new_ty: Index,
) Allocator.Error!Index {
    const old_ty = ip.typeOf(val);
    if (old_ty == new_ty) return val;

    switch (val) {
        .undef => return ip.get(gpa, io, tid, .{ .undef = new_ty }),
        .null_value => {
            if (ip.isOptionalType(new_ty)) return ip.get(gpa, io, tid, .{ .opt = .{
                .ty = new_ty,
                .val = .none,
            } });

            if (ip.isPointerType(new_ty)) switch (ip.indexToKey(new_ty).ptr_type.flags.size) {
                .one, .many, .c => return ip.get(gpa, io, tid, .{ .ptr = .{
                    .ty = new_ty,
                    .base_addr = .int,
                    .byte_offset = 0,
                } }),
                .slice => return ip.get(gpa, io, tid, .{ .slice = .{
                    .ty = new_ty,
                    .ptr = try ip.get(gpa, io, tid, .{ .ptr = .{
                        .ty = ip.slicePtrType(new_ty),
                        .base_addr = .int,
                        .byte_offset = 0,
                    } }),
                    .len = .undef_usize,
                } }),
            };
        },
        else => {
            const unwrapped_val = val.unwrap(ip);
            const val_item = unwrapped_val.getItem(ip);
            switch (val_item.tag) {
                .func_decl => return getCoercedFuncDecl(ip, gpa, io, tid, val, new_ty),
                .func_instance => return getCoercedFuncInstance(ip, gpa, io, tid, val, new_ty),
                .func_coerced => {
                    const func: Index = @enumFromInt(unwrapped_val.getExtra(ip).view().items(.@"0")[
                        val_item.data + std.meta.fieldIndex(Tag.FuncCoerced, "func").?
                    ]);
                    switch (func.unwrap(ip).getTag(ip)) {
                        .func_decl => return getCoercedFuncDecl(ip, gpa, io, tid, val, new_ty),
                        .func_instance => return getCoercedFuncInstance(ip, gpa, io, tid, val, new_ty),
                        else => unreachable,
                    }
                },
                else => {},
            }
        },
    }

    switch (ip.indexToKey(val)) {
        .undef => return ip.get(gpa, io, tid, .{ .undef = new_ty }),
        .func => unreachable,

        .int => |int| switch (ip.indexToKey(new_ty)) {
            .enum_type => return ip.get(gpa, io, tid, .{ .enum_tag = .{
                .ty = new_ty,
                .int = try ip.getCoerced(gpa, io, tid, val, ip.loadEnumType(new_ty).int_tag_type),
            } }),
            .ptr_type => switch (int.storage) {
                inline .u64, .i64 => |int_val| return ip.get(gpa, io, tid, .{ .ptr = .{
                    .ty = new_ty,
                    .base_addr = .int,
                    .byte_offset = @intCast(int_val),
                } }),
                .big_int => unreachable, // must be a usize
            },
            else => if (ip.isIntegerType(new_ty))
                return ip.getCoercedInts(gpa, io, tid, int, new_ty),
        },
        .float => |float| switch (ip.indexToKey(new_ty)) {
            .simple_type => |simple| switch (simple) {
                .f16,
                .f32,
                .f64,
                .f80,
                .f128,
                .c_longdouble,
                .comptime_float,
                => return ip.get(gpa, io, tid, .{ .float = .{
                    .ty = new_ty,
                    .storage = float.storage,
                } }),
                else => {},
            },
            else => {},
        },
        .enum_tag => |enum_tag| if (ip.isIntegerType(new_ty))
            return ip.getCoercedInts(gpa, io, tid, ip.indexToKey(enum_tag.int).int, new_ty),
        .enum_literal => |enum_literal| switch (ip.indexToKey(new_ty)) {
            .enum_type => {
                const enum_type = ip.loadEnumType(new_ty);
                const index = enum_type.nameIndex(ip, enum_literal).?;
                return ip.get(gpa, io, tid, .{ .enum_tag = .{
                    .ty = new_ty,
                    .int = if (enum_type.field_values.len != 0)
                        enum_type.field_values.get(ip)[index]
                    else
                        try ip.get(gpa, io, tid, .{ .int = .{
                            .ty = enum_type.int_tag_type,
                            .storage = .{ .u64 = index },
                        } }),
                } });
            },
            else => {},
        },
        .slice => |slice| if (ip.isPointerType(new_ty) and ip.indexToKey(new_ty).ptr_type.flags.size == .slice)
            return ip.get(gpa, io, tid, .{ .slice = .{
                .ty = new_ty,
                .ptr = try ip.getCoerced(gpa, io, tid, slice.ptr, ip.slicePtrType(new_ty)),
                .len = slice.len,
            } })
        else if (ip.isIntegerType(new_ty))
            return ip.getCoerced(gpa, io, tid, slice.ptr, new_ty),
        .ptr => |ptr| if (ip.isPointerType(new_ty) and ip.indexToKey(new_ty).ptr_type.flags.size != .slice)
            return ip.get(gpa, io, tid, .{ .ptr = .{
                .ty = new_ty,
                .base_addr = ptr.base_addr,
                .byte_offset = ptr.byte_offset,
            } })
        else if (ip.isIntegerType(new_ty))
            switch (ptr.base_addr) {
                .int => return ip.get(gpa, io, tid, .{ .int = .{
                    .ty = .usize_type,
                    .storage = .{ .u64 = @intCast(ptr.byte_offset) },
                } }),
                else => {},
            },
        .opt => |opt| switch (ip.indexToKey(new_ty)) {
            .ptr_type => |ptr_type| return switch (opt.val) {
                .none => switch (ptr_type.flags.size) {
                    .one, .many, .c => try ip.get(gpa, io, tid, .{ .ptr = .{
                        .ty = new_ty,
                        .base_addr = .int,
                        .byte_offset = 0,
                    } }),
                    .slice => try ip.get(gpa, io, tid, .{ .slice = .{
                        .ty = new_ty,
                        .ptr = try ip.get(gpa, io, tid, .{ .ptr = .{
                            .ty = ip.slicePtrType(new_ty),
                            .base_addr = .int,
                            .byte_offset = 0,
                        } }),
                        .len = .undef_usize,
                    } }),
                },
                else => |payload| try ip.getCoerced(gpa, io, tid, payload, new_ty),
            },
            .opt_type => |child_type| return try ip.get(gpa, io, tid, .{ .opt = .{
                .ty = new_ty,
                .val = switch (opt.val) {
                    .none => .none,
                    else => try ip.getCoerced(gpa, io, tid, opt.val, child_type),
                },
            } }),
            else => {},
        },
        .err => |err| if (ip.isErrorSetType(new_ty))
            return ip.get(gpa, io, tid, .{ .err = .{
                .ty = new_ty,
                .name = err.name,
            } })
        else if (ip.isErrorUnionType(new_ty))
            return ip.get(gpa, io, tid, .{ .error_union = .{
                .ty = new_ty,
                .val = .{ .err_name = err.name },
            } }),
        .error_union => |error_union| if (ip.isErrorUnionType(new_ty))
            return ip.get(gpa, io, tid, .{ .error_union = .{
                .ty = new_ty,
                .val = error_union.val,
            } }),
        .aggregate => |aggregate| {
            const new_len: usize = @intCast(ip.aggregateTypeLen(new_ty));
            direct: {
                const old_ty_child = switch (ip.indexToKey(old_ty)) {
                    inline .array_type, .vector_type => |seq_type| seq_type.child,
                    .tuple_type, .struct_type => break :direct,
                    else => unreachable,
                };
                const new_ty_child = switch (ip.indexToKey(new_ty)) {
                    inline .array_type, .vector_type => |seq_type| seq_type.child,
                    .tuple_type, .struct_type => break :direct,
                    else => unreachable,
                };
                if (old_ty_child != new_ty_child) break :direct;
                switch (aggregate.storage) {
                    .bytes => |bytes| return ip.get(gpa, io, tid, .{ .aggregate = .{
                        .ty = new_ty,
                        .storage = .{ .bytes = bytes },
                    } }),
                    .elems => |elems| {
                        const elems_copy = try gpa.dupe(Index, elems[0..new_len]);
                        defer gpa.free(elems_copy);
                        return ip.get(gpa, io, tid, .{ .aggregate = .{
                            .ty = new_ty,
                            .storage = .{ .elems = elems_copy },
                        } });
                    },
                    .repeated_elem => |elem| {
                        return ip.get(gpa, io, tid, .{ .aggregate = .{
                            .ty = new_ty,
                            .storage = .{ .repeated_elem = elem },
                        } });
                    },
                }
            }
            // Direct approach failed - we must recursively coerce elems
            const agg_elems = try gpa.alloc(Index, new_len);
            defer gpa.free(agg_elems);
            // First, fill the vector with the uncoerced elements. We do this to avoid key
            // lifetime issues, since it'll allow us to avoid referencing `aggregate` after we
            // begin interning elems.
            switch (aggregate.storage) {
                .bytes => |bytes| {
                    // We have to intern each value here, so unfortunately we can't easily avoid
                    // the repeated indexToKey calls.
                    for (agg_elems, 0..) |*elem, index| {
                        elem.* = try ip.get(gpa, io, tid, .{ .int = .{
                            .ty = .u8_type,
                            .storage = .{ .u64 = bytes.at(index, ip) },
                        } });
                    }
                },
                .elems => |elems| @memcpy(agg_elems, elems[0..new_len]),
                .repeated_elem => |elem| @memset(agg_elems, elem),
            }
            // Now, coerce each element to its new type.
            for (agg_elems, 0..) |*elem, i| {
                const new_elem_ty = switch (ip.indexToKey(new_ty)) {
                    inline .array_type, .vector_type => |seq_type| seq_type.child,
                    .tuple_type => |tuple_type| tuple_type.types.get(ip)[i],
                    .struct_type => ip.loadStructType(new_ty).field_types.get(ip)[i],
                    else => unreachable,
                };
                elem.* = try ip.getCoerced(gpa, io, tid, elem.*, new_elem_ty);
            }
            return ip.get(gpa, io, tid, .{ .aggregate = .{ .ty = new_ty, .storage = .{ .elems = agg_elems } } });
        },
        else => {},
    }

    switch (ip.indexToKey(new_ty)) {
        .opt_type => |child_type| switch (val) {
            .null_value => return ip.get(gpa, io, tid, .{ .opt = .{
                .ty = new_ty,
                .val = .none,
            } }),
            else => return ip.get(gpa, io, tid, .{ .opt = .{
                .ty = new_ty,
                .val = try ip.getCoerced(gpa, io, tid, val, child_type),
            } }),
        },
        .error_union_type => |error_union_type| return ip.get(gpa, io, tid, .{ .error_union = .{
            .ty = new_ty,
            .val = .{ .payload = try ip.getCoerced(gpa, io, tid, val, error_union_type.payload_type) },
        } }),
        else => {},
    }
    if (std.debug.runtime_safety) {
        std.debug.panic("InternPool.getCoerced of {s} not implemented from {s} to {s}", .{
            @tagName(ip.indexToKey(val)),
            @tagName(ip.indexToKey(old_ty)),
            @tagName(ip.indexToKey(new_ty)),
        });
    }
    unreachable;
}

fn getCoercedFuncDecl(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    val: Index,
    new_ty: Index,
) Allocator.Error!Index {
    const unwrapped_val = val.unwrap(ip);
    const prev_ty: Index = @enumFromInt(unwrapped_val.getExtra(ip).view().items(.@"0")[
        unwrapped_val.getData(ip) + std.meta.fieldIndex(Tag.FuncDecl, "ty").?
    ]);
    if (new_ty == prev_ty) return val;
    return getCoercedFunc(ip, gpa, io, tid, val, new_ty);
}

fn getCoercedFuncInstance(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    val: Index,
    new_ty: Index,
) Allocator.Error!Index {
    const unwrapped_val = val.unwrap(ip);
    const prev_ty: Index = @enumFromInt(unwrapped_val.getExtra(ip).view().items(.@"0")[
        unwrapped_val.getData(ip) + std.meta.fieldIndex(Tag.FuncInstance, "ty").?
    ]);
    if (new_ty == prev_ty) return val;
    return getCoercedFunc(ip, gpa, io, tid, val, new_ty);
}

fn getCoercedFunc(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    func: Index,
    ty: Index,
) Allocator.Error!Index {
    const local = ip.getLocal(tid);
    const items = local.getMutableItems(gpa, io);
    try items.ensureUnusedCapacity(1);
    const extra = local.getMutableExtra(gpa, io);

    const prev_extra_len = extra.mutate.len;
    try extra.ensureUnusedCapacity(@typeInfo(Tag.FuncCoerced).@"struct".fields.len);

    const extra_index = addExtraAssumeCapacity(extra, Tag.FuncCoerced{
        .ty = ty,
        .func = func,
    });
    errdefer extra.mutate.len = prev_extra_len;

    var gop = try ip.getOrPutKey(gpa, io, tid, .{
        .func = ip.extraFuncCoerced(extra.list.*, extra_index),
    });
    defer gop.deinit();
    if (gop == .existing) {
        extra.mutate.len = prev_extra_len;
        return gop.existing;
    }

    items.appendAssumeCapacity(.{
        .tag = .func_coerced,
        .data = extra_index,
    });
    return gop.put();
}

/// Asserts `val` has an integer type.
/// Assumes `new_ty` is an integer type.
pub fn getCoercedInts(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    int: Key.Int,
    new_ty: Index,
) Allocator.Error!Index {
    return ip.get(gpa, io, tid, .{ .int = .{
        .ty = new_ty,
        .storage = int.storage,
    } });
}

pub fn indexToFuncType(ip: *const InternPool, val: Index) ?Key.FuncType {
    const unwrapped_val = val.unwrap(ip);
    const item = unwrapped_val.getItem(ip);
    switch (item.tag) {
        .type_function => return extraFuncType(unwrapped_val.tid, unwrapped_val.getExtra(ip), item.data),
        else => return null,
    }
}

/// includes .comptime_int_type
pub fn isIntegerType(ip: *const InternPool, ty: Index) bool {
    return switch (ty) {
        .usize_type,
        .isize_type,
        .c_char_type,
        .c_short_type,
        .c_ushort_type,
        .c_int_type,
        .c_uint_type,
        .c_long_type,
        .c_ulong_type,
        .c_longlong_type,
        .c_ulonglong_type,
        .comptime_int_type,
        => true,
        else => switch (ty.unwrap(ip).getTag(ip)) {
            .type_int_signed,
            .type_int_unsigned,
            => true,
            else => false,
        },
    };
}

/// does not include .enum_literal_type
pub fn isEnumType(ip: *const InternPool, ty: Index) bool {
    return ip.indexToKey(ty) == .enum_type;
}

pub fn isUnion(ip: *const InternPool, ty: Index) bool {
    return ip.indexToKey(ty) == .union_type;
}

pub fn isFunctionType(ip: *const InternPool, ty: Index) bool {
    return ip.indexToKey(ty) == .func_type;
}

pub fn isPointerType(ip: *const InternPool, ty: Index) bool {
    return ip.indexToKey(ty) == .ptr_type;
}

pub fn isOptionalType(ip: *const InternPool, ty: Index) bool {
    return ip.indexToKey(ty) == .opt_type;
}

/// includes .inferred_error_set_type
pub fn isErrorSetType(ip: *const InternPool, ty: Index) bool {
    return switch (ty) {
        .anyerror_type, .adhoc_inferred_error_set_type => true,
        else => switch (ip.indexToKey(ty)) {
            .error_set_type, .inferred_error_set_type => true,
            else => false,
        },
    };
}

pub fn isInferredErrorSetType(ip: *const InternPool, ty: Index) bool {
    return ty == .adhoc_inferred_error_set_type or ip.indexToKey(ty) == .inferred_error_set_type;
}

pub fn isErrorUnionType(ip: *const InternPool, ty: Index) bool {
    return ip.indexToKey(ty) == .error_union_type;
}

pub fn isAggregateType(ip: *const InternPool, ty: Index) bool {
    return switch (ip.indexToKey(ty)) {
        .array_type, .vector_type, .tuple_type, .struct_type => true,
        else => false,
    };
}

pub fn errorUnionSet(ip: *const InternPool, ty: Index) Index {
    return ip.indexToKey(ty).error_union_type.error_set_type;
}

pub fn errorUnionPayload(ip: *const InternPool, ty: Index) Index {
    return ip.indexToKey(ty).error_union_type.payload_type;
}

/// The is only legal because the initializer is not part of the hash.
pub fn mutateVarInit(ip: *InternPool, io: Io, index: Index, init_index: Index) void {
    const unwrapped_index = index.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    assert(item.tag == .variable);
    @atomicStore(u32, &extra_items[item.data + std.meta.fieldIndex(Tag.Variable, "init").?], @intFromEnum(init_index), .release);
}

pub fn dump(ip: *const InternPool) void {
    var buffer: [4096]u8 = undefined;
    const stderr = std.debug.lockStderr(&buffer);
    defer std.debug.unlockStderr();
    const w = &stderr.file_writer.interface;
    dumpDependencyStatsFallible(ip, w) catch return;
    dumpStatsFallible(ip, w, std.heap.page_allocator) catch return;
    dumpAllFallible(ip, w) catch return;
}

fn dumpDependencyStatsFallible(ip: *const InternPool, w: *Io.Writer) !void {
    const dep_entries_len = ip.dep_entries.items.len - ip.free_dep_entries.items.len;
    const src_hash_deps_len = ip.src_hash_deps.count();
    const nav_val_deps_len = ip.nav_val_deps.count();
    const nav_ty_deps_len = ip.nav_ty_deps.count();
    const func_ies_deps_len = ip.func_ies_deps.count();
    const type_layout_deps_len = ip.type_layout_deps.count();
    const zon_file_deps_len = ip.zon_file_deps.count();
    const embed_file_deps_len = ip.embed_file_deps.count();
    const namespace_deps_len = ip.namespace_deps.count();
    const namespace_name_deps_len = ip.namespace_name_deps.count();
    const dep_entries_size = dep_entries_len * @sizeOf(DepEntry);
    const src_hash_deps_size = src_hash_deps_len * 8;
    const nav_val_deps_size = nav_val_deps_len * 8;
    const nav_ty_deps_size = nav_ty_deps_len * 8;
    const func_ies_deps_size = func_ies_deps_len * 8;
    const type_layout_deps_size = type_layout_deps_len * 8;
    const zon_file_deps_size = zon_file_deps_len * 8;
    const embed_file_deps_size = embed_file_deps_len * 8;
    const namespace_deps_size = namespace_deps_len * 8;
    const namespace_name_deps_size = namespace_name_deps_len * (@sizeOf(NamespaceNameKey) + 4);

    try w.print(
        \\InternPool dependencies: {d} bytes
        \\  {d} entries: {d} bytes
        \\  {d} src_hash: {d} bytes
        \\  {d} nav_val: {d} bytes
        \\  {d} nav_ty: {d} bytes
        \\  {d} func_ies: {d} bytes
        \\  {d} type_layout: {d} bytes
        \\  {d} zon_file: {d} bytes
        \\  {d} embed_file: {d} bytes
        \\  {d} namespace: {d} bytes
        \\  {d} namespace_name: {d} bytes
        \\
    , .{
        dep_entries_size + src_hash_deps_size + nav_val_deps_size + nav_ty_deps_size +
            func_ies_deps_size + type_layout_deps_size + zon_file_deps_size +
            embed_file_deps_size + namespace_deps_size + namespace_name_deps_size,
        dep_entries_len,
        dep_entries_size,
        src_hash_deps_len,
        src_hash_deps_size,
        nav_val_deps_len,
        nav_val_deps_size,
        nav_ty_deps_len,
        nav_ty_deps_size,
        func_ies_deps_len,
        func_ies_deps_size,
        type_layout_deps_len,
        type_layout_deps_size,
        zon_file_deps_len,
        zon_file_deps_size,
        embed_file_deps_len,
        embed_file_deps_size,
        namespace_deps_len,
        namespace_deps_size,
        namespace_name_deps_len,
        namespace_name_deps_size,
    });
}

fn dumpStatsFallible(ip: *const InternPool, w: *Io.Writer, arena: Allocator) !void {
    var items_len: usize = 0;
    var extra_len: usize = 0;
    var limbs_len: usize = 0;
    for (ip.locals) |*local| {
        items_len += local.mutate.items.len;
        extra_len += local.mutate.extra.len;
        limbs_len += local.mutate.limbs.len;
    }
    const items_size = (1 + 4) * items_len;
    const extra_size = 4 * extra_len;
    const limbs_size = 8 * limbs_len;

    // TODO: map overhead size is not taken into account
    const total_size = items_size + extra_size + limbs_size;

    try w.print(
        \\InternPool values: {d} bytes
        \\  {d} items: {d} bytes
        \\  {d} extra: {d} bytes
        \\  {d} limbs: {d} bytes
        \\
    , .{
        total_size,
        items_len,
        items_size,
        extra_len,
        extra_size,
        limbs_len,
        limbs_size,
    });

    const TagStats = struct {
        count: usize = 0,
        bytes: usize = 0,
    };
    var counts = std.AutoArrayHashMap(Tag, TagStats).init(arena);
    for (ip.locals) |*local| {
        // Early check for length 0, because `view()` is invalid if capacity is 0
        if (local.mutate.items.len == 0) continue;
        const items = local.shared.items.view().slice();
        const extra_list = local.shared.extra;
        const extra_items = extra_list.view().items(.@"0");
        for (
            items.items(.tag)[0..local.mutate.items.len],
            items.items(.data)[0..local.mutate.items.len],
        ) |tag, data| {
            const gop = try counts.getOrPut(tag);
            if (!gop.found_existing) gop.value_ptr.* = .{};
            gop.value_ptr.count += 1;
            gop.value_ptr.bytes += 1 + 4 + @as(usize, switch (tag) {
                // Note that in this case, we have technically leaked some extra data
                // bytes which we do not account for here.
                .removed => 0,

                .type_int_signed => 0,
                .type_int_unsigned => 0,
                .type_array_small => @sizeOf(Vector),
                .type_array_big => @sizeOf(Array),
                .type_vector => @sizeOf(Vector),
                .type_pointer => @sizeOf(Tag.TypePointer),
                .type_slice => 0,
                .type_optional => 0,
                .type_anyframe => 0,
                .type_error_union => @sizeOf(Key.ErrorUnionType),
                .type_anyerror_union => 0,
                .type_error_set => b: {
                    const info = extraData(extra_list, Tag.ErrorSet, data);
                    break :b @sizeOf(Tag.ErrorSet) + (@sizeOf(u32) * info.names_len);
                },
                .type_inferred_error_set => 0,
                .type_tuple => b: {
                    const info = extraData(extra_list, TypeTuple, data);
                    break :b @sizeOf(TypeTuple) + (@sizeOf(u32) * 2 * info.fields_len);
                },
                .type_function => b: {
                    const info = extraData(extra_list, Tag.TypeFunction, data);
                    break :b @sizeOf(Tag.TypeFunction) +
                        (@sizeOf(Index) * info.params_len) +
                        (@as(u32, 4) * @intFromBool(info.flags.has_comptime_bits)) +
                        (@as(u32, 4) * @intFromBool(info.flags.has_noalias_bits));
                },

                .type_struct => b: {
                    var n: usize = @typeInfo(Tag.TypeStruct).@"struct".fields.len;
                    const extra = extraDataTrail(extra_list, Tag.TypeStruct, data);
                    switch (extra.data.flags.any_captures) {
                        .reified => n += 2, // type_hash: PackedU64
                        .true => {
                            n += 1; // captures_len: u32
                            n += extra_items[extra.end]; // capture: CaptureValue
                        },
                        .false => {},
                    }
                    n += extra.data.fields_len; // field_name: NullTerminatedString
                    n += extra.data.fields_len; // field_type: Index
                    if (extra.data.flags.any_field_defaults) {
                        n += extra.data.fields_len; // field_default: Index
                    }
                    if (extra.data.flags.any_field_aligns) {
                        n += (extra.data.fields_len + 3) / 4; // field_align: Alignment
                    }
                    if (extra.data.flags.any_comptime_fields) {
                        n += (extra.data.fields_len + 31) / 32; // field_is_comptime_bits: u32
                    }
                    if (extra.data.flags.layout == .auto) {
                        n += extra.data.fields_len; // field_runtime_order: RuntimeOrder
                    }
                    n += extra.data.fields_len; // field_offset: u32
                    break :b n * @sizeOf(u32);
                },
                .type_struct_packed_auto, .type_struct_packed_explicit => b: {
                    var n: usize = @typeInfo(Tag.TypeStructPacked).@"struct".fields.len;
                    const extra = extraDataTrail(extra_list, Tag.TypeStructPacked, data);
                    switch (extra.data.bits.captures_len) {
                        .reified => n += 2, // type_hash: PackedU64
                        _ => |len| n += @intFromEnum(len), // capture: CaptureValue
                    }
                    n += extra.data.fields_len; // field_name: NullTerminatedString
                    n += extra.data.fields_len; // field_type: Index
                    break :b n * @sizeOf(u32);
                },
                .type_struct_packed_auto_defaults, .type_struct_packed_explicit_defaults => b: {
                    var n: usize = @typeInfo(Tag.TypeStructPacked).@"struct".fields.len;
                    const extra = extraDataTrail(extra_list, Tag.TypeStructPacked, data);
                    switch (extra.data.bits.captures_len) {
                        .reified => n += 2, // type_hash: PackedU64
                        _ => |len| n += @intFromEnum(len), // capture: CaptureValue
                    }
                    n += extra.data.fields_len; // field_name: NullTerminatedString
                    n += extra.data.fields_len; // field_type: Index
                    n += extra.data.fields_len; // field_default: Index
                    break :b n * @sizeOf(u32);
                },
                .type_union => b: {
                    var n: usize = @typeInfo(Tag.TypeUnion).@"struct".fields.len;
                    const extra = extraDataTrail(extra_list, Tag.TypeUnion, data);
                    switch (extra.data.flags.any_captures) {
                        .reified => n += 2, // type_hash: PackedU64
                        .true => {
                            n += 1; // captures_len: u32
                            n += extra_items[extra.end]; // capture: CaptureValue
                        },
                        .false => {},
                    }
                    n += extra.data.fields_len; // field_type: Index
                    if (extra.data.flags.any_field_aligns) {
                        n += (extra.data.fields_len + 3) / 4; // field_align: Alignment
                    }
                    break :b n * @sizeOf(u32);
                },
                .type_union_packed_auto, .type_union_packed_explicit => b: {
                    var n: usize = @typeInfo(Tag.TypeUnionPacked).@"struct".fields.len;
                    const extra = extraDataTrail(extra_list, Tag.TypeUnionPacked, data);
                    switch (extra.data.bits.captures_len) {
                        .reified => n += 2, // type_hash: PackedU64
                        _ => |len| n += @intFromEnum(len), // capture: CaptureValue
                    }
                    n += extra.data.fields_len; // field_type: Index
                    break :b n * @sizeOf(u32);
                },
                .type_enum_auto => b: {
                    var n: usize = @typeInfo(Tag.TypeEnum).@"struct".fields.len;
                    const extra = extraData(extra_list, Tag.TypeEnum, data);
                    switch (extra.bits.captures_len) {
                        .generated_union_tag => n += 1, // owner_union: Index
                        .reified => {
                            n += 1; // zir_index: TrackedInst.Index,
                            n += 2; // type_hash: PackedU64
                        },
                        _ => |len| {
                            n += 1; // zir_index: TrackedInst.Index,
                            n += @intFromEnum(len); // capture: CaptureValue
                        },
                    }
                    n += extra.fields_len; // field_name: NullTerminatedString
                    break :b n * @sizeOf(u32);
                },
                .type_enum_explicit, .type_enum_nonexhaustive => b: {
                    var n: usize = @typeInfo(Tag.TypeEnum).@"struct".fields.len;
                    const extra = extraData(extra_list, Tag.TypeEnum, data);
                    switch (extra.bits.captures_len) {
                        .generated_union_tag => n += 1, // owner_union: Index
                        .reified => {
                            n += 1; // zir_index: TrackedInst.Index,
                            n += 2; // type_hash: PackedU64
                        },
                        _ => |len| {
                            n += 1; // zir_index: TrackedInst.Index,
                            n += @intFromEnum(len); // capture: CaptureValue
                        },
                    }
                    n += 1; // field_value_map: MapIndex
                    n += extra.fields_len; // field_name: NullTerminatedString
                    n += extra.fields_len; // field_value: Index
                    break :b n * @sizeOf(u32);
                },
                .type_opaque => b: {
                    var n: usize = @typeInfo(Tag.TypeEnum).@"struct".fields.len;
                    const extra = extraData(extra_list, Tag.TypeOpaque, data);
                    n += extra.captures_len; // capture: CaptureValue
                    break :b n * @sizeOf(u32);
                },

                .undef => 0,
                .simple_type => 0,
                .simple_value => 0,
                .ptr_nav => @sizeOf(PtrNav),
                .ptr_comptime_alloc => @sizeOf(PtrComptimeAlloc),
                .ptr_uav => @sizeOf(PtrUav),
                .ptr_uav_aligned => @sizeOf(PtrUavAligned),
                .ptr_comptime_field => @sizeOf(PtrComptimeField),
                .ptr_int => @sizeOf(PtrInt),
                .ptr_eu_payload => @sizeOf(PtrBase),
                .ptr_opt_payload => @sizeOf(PtrBase),
                .ptr_elem => @sizeOf(PtrBaseIndex),
                .ptr_field => @sizeOf(PtrBaseIndex),
                .ptr_slice => @sizeOf(PtrSlice),
                .opt_null => 0,
                .opt_payload => @sizeOf(Tag.TypeValue),
                .int_u8 => 0,
                .int_u16 => 0,
                .int_u32 => 0,
                .int_i32 => 0,
                .int_usize => 0,
                .int_comptime_int_u32 => 0,
                .int_comptime_int_i32 => 0,
                .int_small => @sizeOf(IntSmall),

                .int_positive,
                .int_negative,
                => b: {
                    const limbs_list = local.shared.getLimbs();
                    const int: Int = @bitCast(limbs_list.view().items(.@"0")[data..][0..Int.limbs_items_len].*);
                    break :b @sizeOf(Int) + int.limbs_len * @sizeOf(Limb);
                },

                .error_set_error, .error_union_error => @sizeOf(Key.Error),
                .error_union_payload => @sizeOf(Tag.TypeValue),
                .enum_literal => 0,
                .enum_tag => @sizeOf(Tag.EnumTag),

                .bytes => b: {
                    const info = extraData(extra_list, Bytes, data);
                    const len: usize = @intCast(ip.aggregateTypeLenIncludingSentinel(info.ty));
                    break :b @sizeOf(Bytes) + len + @intFromBool(info.bytes.at(len - 1, ip) != 0);
                },
                .aggregate => b: {
                    const info = extraData(extra_list, Tag.Aggregate, data);
                    const fields_len: u32 = @intCast(ip.aggregateTypeLenIncludingSentinel(info.ty));
                    break :b @sizeOf(Tag.Aggregate) + (@sizeOf(Index) * fields_len);
                },
                .repeated => @sizeOf(Repeated),

                .float_f16 => 0,
                .float_f32 => 0,
                .float_f64 => @sizeOf(Float64),
                .float_f80 => @sizeOf(Float80),
                .float_f128 => @sizeOf(Float128),
                .float_c_longdouble_f80 => @sizeOf(Float80),
                .float_c_longdouble_f128 => @sizeOf(Float128),
                .float_comptime_float => @sizeOf(Float128),
                .variable, .threadlocal_variable => @sizeOf(Tag.Variable),
                .@"extern" => @sizeOf(Tag.Extern),
                .func_decl => @sizeOf(Tag.FuncDecl),
                .func_instance => b: {
                    const info = extraData(extra_list, Tag.FuncInstance, data);
                    const ty = ip.typeOf(info.generic_owner);
                    const params_len = ip.indexToKey(ty).func_type.param_types.len;
                    break :b @sizeOf(Tag.FuncInstance) + @sizeOf(Index) * params_len;
                },
                .func_coerced => @sizeOf(Tag.FuncCoerced),
                .only_possible_value => 0,
                .union_value => @sizeOf(Key.Union),
                .bitpack => 2 * @sizeOf(u32),

                .memoized_call => b: {
                    const info = extraData(extra_list, MemoizedCall, data);
                    break :b @sizeOf(MemoizedCall) + (@sizeOf(Index) * info.args_len);
                },
            });
        }
    }
    const SortContext = struct {
        map: *std.AutoArrayHashMap(Tag, TagStats),
        pub fn lessThan(ctx: @This(), a_index: usize, b_index: usize) bool {
            const values = ctx.map.values();
            return values[a_index].bytes > values[b_index].bytes;
            //return values[a_index].count > values[b_index].count;
        }
    };
    counts.sort(SortContext{ .map = &counts });
    const len = @min(50, counts.count());
    try w.print("  top 50 tags:\n", .{});
    for (counts.keys()[0..len], counts.values()[0..len]) |tag, stats| {
        try w.print("    {t}: {d} occurrences, {d} total bytes\n", .{ tag, stats.count, stats.bytes });
    }
}

fn dumpAllFallible(ip: *const InternPool, w: *Io.Writer) anyerror!void {
    for (ip.locals, 0..) |*local, tid| {
        // Early check for length 0, because `view()` is invalid if capacity is 0
        if (local.mutate.items.len == 0) continue;
        const items = local.shared.items.view();
        for (
            items.items(.tag)[0..local.mutate.items.len],
            items.items(.data)[0..local.mutate.items.len],
            0..,
        ) |tag, data, index| {
            const i = Index.Unwrapped.wrap(.{ .tid = @enumFromInt(tid), .index = @intCast(index) }, ip);
            try w.print("${d} = {s}(", .{ i, @tagName(tag) });
            switch (tag) {
                .removed => {},

                .simple_type => try w.print("{s}", .{@tagName(@as(SimpleType, @enumFromInt(@intFromEnum(i))))}),
                .simple_value => try w.print("{s}", .{@tagName(@as(SimpleValue, @enumFromInt(@intFromEnum(i))))}),

                .type_int_signed,
                .type_int_unsigned,
                .type_array_small,
                .type_array_big,
                .type_vector,
                .type_pointer,
                .type_optional,
                .type_anyframe,
                .type_error_union,
                .type_anyerror_union,
                .type_error_set,
                .type_inferred_error_set,
                .type_tuple,
                .type_function,
                .type_struct,
                .type_struct_packed_auto,
                .type_struct_packed_explicit,
                .type_struct_packed_auto_defaults,
                .type_struct_packed_explicit_defaults,
                .type_union,
                .type_union_packed_auto,
                .type_union_packed_explicit,
                .type_enum_auto,
                .type_enum_explicit,
                .type_enum_nonexhaustive,
                .type_opaque,
                .undef,
                .ptr_nav,
                .ptr_comptime_alloc,
                .ptr_uav,
                .ptr_uav_aligned,
                .ptr_comptime_field,
                .ptr_int,
                .ptr_eu_payload,
                .ptr_opt_payload,
                .ptr_elem,
                .ptr_field,
                .ptr_slice,
                .opt_payload,
                .int_u8,
                .int_u16,
                .int_u32,
                .int_i32,
                .int_usize,
                .int_comptime_int_u32,
                .int_comptime_int_i32,
                .int_small,
                .int_positive,
                .int_negative,
                .error_set_error,
                .error_union_error,
                .error_union_payload,
                .enum_literal,
                .enum_tag,
                .bytes,
                .aggregate,
                .repeated,
                .float_f16,
                .float_f32,
                .float_f64,
                .float_f80,
                .float_f128,
                .float_c_longdouble_f80,
                .float_c_longdouble_f128,
                .float_comptime_float,
                .variable,
                .threadlocal_variable,
                .@"extern",
                .func_decl,
                .func_instance,
                .func_coerced,
                .union_value,
                .bitpack,
                .memoized_call,
                => try w.print("{d}", .{data}),

                .opt_null,
                .type_slice,
                .only_possible_value,
                => try w.print("${d}", .{data}),
            }
            try w.writeAll(")\n");
        }
    }
}

pub fn dumpGenericInstances(ip: *const InternPool, allocator: Allocator) void {
    var buffer: [4096]u8 = undefined;
    const stderr = std.debug.lockStderr(&buffer);
    defer std.debug.unlockStderr();
    const w = &stderr.file_writer.interface;
    ip.dumpGenericInstancesFallible(allocator, w) catch return;
}

pub fn dumpGenericInstancesFallible(ip: *const InternPool, allocator: Allocator, w: *Io.Writer) !void {
    var arena_allocator = std.heap.ArenaAllocator.init(allocator);
    defer arena_allocator.deinit();
    const arena = arena_allocator.allocator();

    var instances: std.AutoArrayHashMapUnmanaged(Index, std.ArrayList(Index)) = .empty;
    for (ip.locals, 0..) |*local, tid| {
        const items = local.shared.items.view().slice();
        const extra_list = local.shared.extra;
        for (
            items.items(.tag)[0..local.mutate.items.len],
            items.items(.data)[0..local.mutate.items.len],
            0..,
        ) |tag, data, index| {
            if (tag != .func_instance) continue;
            const info = extraData(extra_list, Tag.FuncInstance, data);

            const gop = try instances.getOrPut(arena, info.generic_owner);
            if (!gop.found_existing) gop.value_ptr.* = .empty;

            try gop.value_ptr.append(
                arena,
                Index.Unwrapped.wrap(.{ .tid = @enumFromInt(tid), .index = @intCast(index) }, ip),
            );
        }
    }

    const SortContext = struct {
        values: []std.ArrayList(Index),
        pub fn lessThan(ctx: @This(), a_index: usize, b_index: usize) bool {
            return ctx.values[a_index].items.len > ctx.values[b_index].items.len;
        }
    };

    instances.sort(SortContext{ .values = instances.values() });
    var it = instances.iterator();
    while (it.next()) |entry| {
        const generic_fn_owner_nav = ip.getNav(ip.funcDeclInfo(entry.key_ptr.*).owner_nav);
        try w.print("{f} ({d}): \n", .{ generic_fn_owner_nav.name.fmt(ip), entry.value_ptr.items.len });
        for (entry.value_ptr.items) |index| {
            const unwrapped_index = index.unwrap(ip);
            const func = ip.extraFuncInstance(unwrapped_index.tid, unwrapped_index.getExtra(ip), unwrapped_index.getData(ip));
            const owner_nav = ip.getNav(func.owner_nav);
            try w.print("  {f}: (", .{owner_nav.name.fmt(ip)});
            for (func.comptime_args.get(ip)) |arg| {
                if (arg != .none) {
                    const key = ip.indexToKey(arg);
                    try w.print(" {} ", .{key});
                }
            }
            try w.writeAll(")\n");
        }
    }
}

pub fn getNav(ip: *const InternPool, index: Nav.Index) Nav {
    const unwrapped = index.unwrap(ip);
    const navs = ip.getLocalShared(unwrapped.tid).navs.acquire();
    return navs.view().get(unwrapped.index).unpack();
}

pub fn namespacePtr(ip: *InternPool, namespace_index: NamespaceIndex) *Zcu.Namespace {
    const unwrapped_namespace_index = namespace_index.unwrap(ip);
    const namespaces = ip.getLocalShared(unwrapped_namespace_index.tid).namespaces.acquire();
    const namespaces_bucket = namespaces.view().items(.@"0")[unwrapped_namespace_index.bucket_index];
    return &namespaces_bucket[unwrapped_namespace_index.index];
}

/// Create a `ComptimeUnit`, forming an `AnalUnit` for a `comptime` declaration.
pub fn createComptimeUnit(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    zir_index: TrackedInst.Index,
    namespace: NamespaceIndex,
) Allocator.Error!ComptimeUnit.Id {
    const comptime_units = ip.getLocal(tid).getMutableComptimeUnits(gpa, io);
    const id_unwrapped: ComptimeUnit.Id.Unwrapped = .{
        .tid = tid,
        .index = comptime_units.mutate.len,
    };
    try comptime_units.append(.{.{
        .zir_index = zir_index,
        .namespace = namespace,
    }});
    return id_unwrapped.wrap(ip);
}

pub fn getComptimeUnit(ip: *const InternPool, id: ComptimeUnit.Id) ComptimeUnit {
    const unwrapped = id.unwrap(ip);
    const comptime_units = ip.getLocalShared(unwrapped.tid).comptime_units.acquire();
    return comptime_units.view().items(.@"0")[unwrapped.index];
}

/// Create a `Nav` which does not undergo semantic analysis.
/// Since it is never analyzed, the `Nav`'s value must be known at creation time.
fn createNav(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    opts: struct {
        name: NullTerminatedString,
        fqn: NullTerminatedString,
        val: InternPool.Index,
        is_const: bool,
        alignment: Alignment,
        @"linksection": OptionalNullTerminatedString,
        @"addrspace": std.builtin.AddressSpace,
    },
) Allocator.Error!Nav.Index {
    const navs = ip.getLocal(tid).getMutableNavs(gpa, io);
    const index_unwrapped: Nav.Index.Unwrapped = .{
        .tid = tid,
        .index = navs.mutate.len,
    };
    try navs.append(Nav.pack(.{
        .name = opts.name,
        .fqn = opts.fqn,
        .analysis = null,
        .status = .{ .fully_resolved = .{
            .val = opts.val,
            .is_const = opts.is_const,
            .alignment = opts.alignment,
            .@"linksection" = opts.@"linksection",
            .@"addrspace" = opts.@"addrspace",
        } },
    }));
    return index_unwrapped.wrap(ip);
}

/// Create a `Nav` which undergoes semantic analysis because it corresponds to a source declaration.
/// The value of the `Nav` is initially unresolved.
pub fn createDeclNav(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    name: NullTerminatedString,
    fqn: NullTerminatedString,
    zir_index: TrackedInst.Index,
    namespace: NamespaceIndex,
) Allocator.Error!Nav.Index {
    const navs = ip.getLocal(tid).getMutableNavs(gpa, io);

    try navs.ensureUnusedCapacity(1);

    const nav = Nav.Index.Unwrapped.wrap(.{
        .tid = tid,
        .index = navs.mutate.len,
    }, ip);

    navs.appendAssumeCapacity(Nav.pack(.{
        .name = name,
        .fqn = fqn,
        .analysis = .{
            .namespace = namespace,
            .zir_index = zir_index,
            .wanted = false,
        },
        .status = .unresolved,
    }));

    return nav;
}

/// Resolve the type of a `Nav` with an analysis owner.
/// If its status is already `resolved`, the old value is discarded.
pub fn resolveNavType(
    ip: *InternPool,
    io: Io,
    nav: Nav.Index,
    resolved: struct {
        type: InternPool.Index,
        is_const: bool,
        alignment: Alignment,
        @"linksection": OptionalNullTerminatedString,
        @"addrspace": std.builtin.AddressSpace,
        is_threadlocal: bool,
        is_extern_decl: bool,
    },
) void {
    const unwrapped = nav.unwrap(ip);

    const local = ip.getLocal(unwrapped.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const navs = local.shared.navs.view();

    const nav_analysis_namespace = navs.items(.analysis_namespace);
    const nav_analysis_zir_index = navs.items(.analysis_zir_index);
    const nav_types = navs.items(.type_or_val);
    const nav_linksections = navs.items(.@"linksection");
    const nav_bits = navs.items(.bits);

    assert(nav_analysis_namespace[unwrapped.index] != .none);
    assert(nav_analysis_zir_index[unwrapped.index] != .none);

    @atomicStore(InternPool.Index, &nav_types[unwrapped.index], resolved.type, .release);
    @atomicStore(OptionalNullTerminatedString, &nav_linksections[unwrapped.index], resolved.@"linksection", .release);

    var bits = nav_bits[unwrapped.index];
    bits.status = if (resolved.is_extern_decl) .type_resolved_extern_decl else .type_resolved;
    bits.is_const = resolved.is_const;
    bits.alignment = resolved.alignment;
    bits.@"addrspace" = resolved.@"addrspace";
    bits.is_threadlocal = resolved.is_threadlocal;
    @atomicStore(Nav.Repr.Bits, &nav_bits[unwrapped.index], bits, .release);
}

/// Resolve the value of a `Nav` with an analysis owner.
/// If its status is already `resolved`, the old value is discarded.
pub fn resolveNavValue(
    ip: *InternPool,
    io: Io,
    nav: Nav.Index,
    resolved: struct {
        val: InternPool.Index,
        is_const: bool,
        alignment: Alignment,
        @"linksection": OptionalNullTerminatedString,
        @"addrspace": std.builtin.AddressSpace,
    },
) void {
    const unwrapped = nav.unwrap(ip);

    const local = ip.getLocal(unwrapped.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const navs = local.shared.navs.view();

    const nav_analysis_namespace = navs.items(.analysis_namespace);
    const nav_analysis_zir_index = navs.items(.analysis_zir_index);
    const nav_vals = navs.items(.type_or_val);
    const nav_linksections = navs.items(.@"linksection");
    const nav_bits = navs.items(.bits);

    assert(nav_analysis_namespace[unwrapped.index] != .none);
    assert(nav_analysis_zir_index[unwrapped.index] != .none);

    @atomicStore(InternPool.Index, &nav_vals[unwrapped.index], resolved.val, .release);
    @atomicStore(OptionalNullTerminatedString, &nav_linksections[unwrapped.index], resolved.@"linksection", .release);

    var bits = nav_bits[unwrapped.index];
    bits.status = .fully_resolved;
    bits.is_const = resolved.is_const;
    bits.alignment = resolved.alignment;
    bits.@"addrspace" = resolved.@"addrspace";
    @atomicStore(Nav.Repr.Bits, &nav_bits[unwrapped.index], bits, .release);
}

pub fn createNamespace(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    initialization: Zcu.Namespace,
) Allocator.Error!NamespaceIndex {
    const local = ip.getLocal(tid);
    const free_list_next = local.mutate.namespaces.free_list;
    if (free_list_next != Local.BucketListMutate.free_list_sentinel) {
        const reused_namespace_index: NamespaceIndex = @enumFromInt(free_list_next);
        const reused_namespace = ip.namespacePtr(reused_namespace_index);
        local.mutate.namespaces.free_list =
            @intFromEnum(@field(reused_namespace, Local.namespace_next_free_field));
        reused_namespace.* = initialization;
        return reused_namespace_index;
    }
    const namespaces = local.getMutableNamespaces(gpa, io);
    const last_bucket_len = local.mutate.namespaces.last_bucket_len & Local.namespaces_bucket_mask;
    if (last_bucket_len == 0) {
        try namespaces.ensureUnusedCapacity(1);
        var arena = namespaces.arena.promote(namespaces.gpa);
        defer namespaces.arena.* = arena.state;
        namespaces.appendAssumeCapacity(.{try arena.allocator().create(
            [1 << Local.namespaces_bucket_width]Zcu.Namespace,
        )});
    }
    const unwrapped_namespace_index: NamespaceIndex.Unwrapped = .{
        .tid = tid,
        .bucket_index = namespaces.mutate.len - 1,
        .index = last_bucket_len,
    };
    local.mutate.namespaces.last_bucket_len = last_bucket_len + 1;
    const namespace_index = unwrapped_namespace_index.wrap(ip);
    ip.namespacePtr(namespace_index).* = initialization;
    return namespace_index;
}

pub fn destroyNamespace(
    ip: *InternPool,
    tid: Zcu.PerThread.Id,
    namespace_index: NamespaceIndex,
) void {
    const local = ip.getLocal(tid);
    const namespace = ip.namespacePtr(namespace_index);
    namespace.* = .{
        .parent = undefined,
        .file_scope = undefined,
        .owner_type = undefined,
        .generation = undefined,
    };
    @field(namespace, Local.namespace_next_free_field) =
        @enumFromInt(local.mutate.namespaces.free_list);
    local.mutate.namespaces.free_list = @intFromEnum(namespace_index);
}

pub fn filePtr(ip: *const InternPool, file_index: FileIndex) *Zcu.File {
    const file_index_unwrapped = file_index.unwrap(ip);
    const files = ip.getLocalShared(file_index_unwrapped.tid).files.acquire();
    return files.view().items(.file)[file_index_unwrapped.index];
}

pub fn createFile(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    file: File,
) Allocator.Error!FileIndex {
    const files = ip.getLocal(tid).getMutableFiles(gpa, io);
    const file_index_unwrapped: FileIndex.Unwrapped = .{
        .tid = tid,
        .index = files.mutate.len,
    };
    try files.append(file);
    return file_index_unwrapped.wrap(ip);
}

const EmbeddedNulls = enum {
    no_embedded_nulls,
    maybe_embedded_nulls,

    fn StringType(comptime embedded_nulls: EmbeddedNulls) type {
        return switch (embedded_nulls) {
            .no_embedded_nulls => NullTerminatedString,
            .maybe_embedded_nulls => String,
        };
    }

    fn OptionalStringType(comptime embedded_nulls: EmbeddedNulls) type {
        return switch (embedded_nulls) {
            .no_embedded_nulls => OptionalNullTerminatedString,
            .maybe_embedded_nulls => OptionalString,
        };
    }
};

pub fn getOrPutString(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    slice: []const u8,
    comptime embedded_nulls: EmbeddedNulls,
) Allocator.Error!embedded_nulls.StringType() {
    const string_bytes = ip.getLocal(tid).getMutableStringBytes(gpa, io);
    try string_bytes.ensureUnusedCapacity(slice.len + 1);
    string_bytes.appendSliceAssumeCapacity(.{slice});
    string_bytes.appendAssumeCapacity(.{0});
    return ip.getOrPutTrailingString(gpa, io, tid, @intCast(slice.len + 1), embedded_nulls);
}

pub fn getOrPutStringFmt(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    comptime format: []const u8,
    args: anytype,
    comptime embedded_nulls: EmbeddedNulls,
) Allocator.Error!embedded_nulls.StringType() {
    // ensure that references to strings in args do not get invalidated
    const format_z = format ++ .{0};
    const len: u32 = @intCast(std.fmt.count(format_z, args));
    const string_bytes = ip.getLocal(tid).getMutableStringBytes(gpa, io);
    const slice = try string_bytes.addManyAsSlice(len);
    assert((std.fmt.bufPrint(slice[0], format_z, args) catch unreachable).len == len);
    return ip.getOrPutTrailingString(gpa, io, tid, len, embedded_nulls);
}

pub fn getOrPutStringOpt(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    slice: ?[]const u8,
    comptime embedded_nulls: EmbeddedNulls,
) Allocator.Error!embedded_nulls.OptionalStringType() {
    const string = try getOrPutString(ip, gpa, io, tid, slice orelse return .none, embedded_nulls);
    return string.toOptional();
}

/// Uses the last len bytes of strings as the key.
pub fn getOrPutTrailingString(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    len: u32,
    comptime embedded_nulls: EmbeddedNulls,
) Allocator.Error!embedded_nulls.StringType() {
    const local = ip.getLocal(tid);
    const strings = local.getMutableStrings(gpa, io);
    try strings.ensureUnusedCapacity(1);
    const string_bytes = local.getMutableStringBytes(gpa, io);
    const start: u32 = @intCast(string_bytes.mutate.len - len);
    if (len > 0 and string_bytes.view().items(.@"0")[string_bytes.mutate.len - 1] == 0) {
        string_bytes.mutate.len -= 1;
    } else {
        try string_bytes.ensureUnusedCapacity(1);
    }
    const key: []const u8 = string_bytes.view().items(.@"0")[start..];
    const value: embedded_nulls.StringType() = @enumFromInt(@intFromEnum((String.Unwrapped{
        .tid = tid,
        .index = strings.mutate.len - 1,
    }).wrap(ip)));
    const has_embedded_null = std.mem.indexOfScalar(u8, key, 0) != null;
    switch (embedded_nulls) {
        .no_embedded_nulls => assert(!has_embedded_null),
        .maybe_embedded_nulls => if (has_embedded_null) {
            string_bytes.appendAssumeCapacity(.{0});
            strings.appendAssumeCapacity(.{string_bytes.mutate.len});
            return value;
        },
    }

    const full_hash = Hash.hash(0, key);
    const hash: u32 = @truncate(full_hash >> 32);
    const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
    var map = shard.shared.string_map.acquire();
    const Map = @TypeOf(map);
    var map_mask = map.header().mask();
    var map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.acquire().unwrap() orelse break;
        if (entry.hash != hash) continue;
        if (!index.eqlSlice(key, ip)) continue;
        string_bytes.shrinkRetainingCapacity(start);
        return @enumFromInt(@intFromEnum(index));
    }
    shard.mutate.string_map.mutex.lock(io, tid);
    defer shard.mutate.string_map.mutex.unlock(io);
    if (map.entries != shard.shared.string_map.entries) {
        map = shard.shared.string_map;
        map_mask = map.header().mask();
        map_index = hash;
    }
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.acquire().unwrap() orelse break;
        if (entry.hash != hash) continue;
        if (!index.eqlSlice(key, ip)) continue;
        string_bytes.shrinkRetainingCapacity(start);
        return @enumFromInt(@intFromEnum(index));
    }
    defer shard.mutate.string_map.len += 1;
    const map_header = map.header().*;
    if (shard.mutate.string_map.len < map_header.capacity * 3 / 5) {
        string_bytes.appendAssumeCapacity(.{0});
        strings.appendAssumeCapacity(.{string_bytes.mutate.len});
        const entry = &map.entries[map_index];
        entry.hash = hash;
        entry.release(@enumFromInt(@intFromEnum(value)));
        return value;
    }
    const arena_state = &local.mutate.arena;
    var arena = arena_state.promote(gpa);
    defer arena_state.* = arena.state;
    const new_map_capacity = map_header.capacity * 2;
    const new_map_buf = try arena.allocator().alignedAlloc(
        u8,
        .fromByteUnits(Map.alignment),
        Map.entries_offset + new_map_capacity * @sizeOf(Map.Entry),
    );
    const new_map: Map = .{ .entries = @ptrCast(new_map_buf[Map.entries_offset..].ptr) };
    new_map.header().* = .{ .capacity = new_map_capacity };
    @memset(new_map.entries[0..new_map_capacity], .{ .value = .none, .hash = undefined });
    const new_map_mask = new_map.header().mask();
    map_index = 0;
    while (map_index < map_header.capacity) : (map_index += 1) {
        const entry = &map.entries[map_index];
        const index = entry.value.unwrap() orelse continue;
        const item_hash = entry.hash;
        var new_map_index = item_hash;
        while (true) : (new_map_index += 1) {
            new_map_index &= new_map_mask;
            const new_entry = &new_map.entries[new_map_index];
            if (new_entry.value != .none) continue;
            new_entry.* = .{
                .value = index.toOptional(),
                .hash = item_hash,
            };
            break;
        }
    }
    map = new_map;
    map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= new_map_mask;
        if (map.entries[map_index].value == .none) break;
    }
    string_bytes.appendAssumeCapacity(.{0});
    strings.appendAssumeCapacity(.{string_bytes.mutate.len});
    map.entries[map_index] = .{
        .value = @enumFromInt(@intFromEnum(value)),
        .hash = hash,
    };
    shard.shared.string_map.release(new_map);
    return value;
}

pub fn getString(ip: *InternPool, key: []const u8) OptionalNullTerminatedString {
    const full_hash = Hash.hash(0, key);
    const hash: u32 = @truncate(full_hash >> 32);
    const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
    const map = shard.shared.string_map.acquire();
    const map_mask = map.header().mask();
    var map_index = hash;
    while (true) : (map_index += 1) {
        map_index &= map_mask;
        const entry = &map.entries[map_index];
        const index = entry.value.unwrap() orelse return .none;
        if (entry.hash != hash) continue;
        if (index.eqlSlice(key, ip)) return index.toOptional();
    }
}

pub fn typeOf(ip: *const InternPool, index: Index) Index {
    // This optimization of static keys is required so that typeOf can be called
    // on static keys that haven't been added yet during static key initialization.
    // An alternative would be to topological sort the static keys, but this would
    // mean that the range of type indices would not be dense.
    return switch (index) {
        .u0_type,
        .i0_type,
        .u1_type,
        .u8_type,
        .i8_type,
        .u16_type,
        .i16_type,
        .u29_type,
        .u32_type,
        .i32_type,
        .u64_type,
        .i64_type,
        .u80_type,
        .u128_type,
        .i128_type,
        .u256_type,
        .usize_type,
        .isize_type,
        .c_char_type,
        .c_short_type,
        .c_ushort_type,
        .c_int_type,
        .c_uint_type,
        .c_long_type,
        .c_ulong_type,
        .c_longlong_type,
        .c_ulonglong_type,
        .c_longdouble_type,
        .f16_type,
        .f32_type,
        .f64_type,
        .f80_type,
        .f128_type,
        .anyopaque_type,
        .bool_type,
        .void_type,
        .type_type,
        .anyerror_type,
        .comptime_int_type,
        .comptime_float_type,
        .noreturn_type,
        .anyframe_type,
        .null_type,
        .undefined_type,
        .enum_literal_type,
        .ptr_usize_type,
        .ptr_const_comptime_int_type,
        .manyptr_u8_type,
        .manyptr_const_u8_type,
        .manyptr_const_u8_sentinel_0_type,
        .manyptr_const_slice_const_u8_type,
        .slice_const_u8_type,
        .slice_const_u8_sentinel_0_type,
        .slice_const_slice_const_u8_type,
        .optional_type_type,
        .manyptr_const_type_type,
        .slice_const_type_type,
        .vector_8_i8_type,
        .vector_16_i8_type,
        .vector_32_i8_type,
        .vector_64_i8_type,
        .vector_1_u8_type,
        .vector_2_u8_type,
        .vector_4_u8_type,
        .vector_8_u8_type,
        .vector_16_u8_type,
        .vector_32_u8_type,
        .vector_64_u8_type,
        .vector_2_i16_type,
        .vector_4_i16_type,
        .vector_8_i16_type,
        .vector_16_i16_type,
        .vector_32_i16_type,
        .vector_4_u16_type,
        .vector_8_u16_type,
        .vector_16_u16_type,
        .vector_32_u16_type,
        .vector_2_i32_type,
        .vector_4_i32_type,
        .vector_8_i32_type,
        .vector_16_i32_type,
        .vector_4_u32_type,
        .vector_8_u32_type,
        .vector_16_u32_type,
        .vector_2_i64_type,
        .vector_4_i64_type,
        .vector_8_i64_type,
        .vector_2_u64_type,
        .vector_4_u64_type,
        .vector_8_u64_type,
        .vector_1_u128_type,
        .vector_2_u128_type,
        .vector_1_u256_type,
        .vector_4_f16_type,
        .vector_8_f16_type,
        .vector_16_f16_type,
        .vector_32_f16_type,
        .vector_2_f32_type,
        .vector_4_f32_type,
        .vector_8_f32_type,
        .vector_16_f32_type,
        .vector_2_f64_type,
        .vector_4_f64_type,
        .vector_8_f64_type,
        .optional_noreturn_type,
        .anyerror_void_error_union_type,
        .adhoc_inferred_error_set_type,
        .generic_poison_type,
        .empty_tuple_type,
        => .type_type,

        .undef => .undefined_type,
        .zero, .one, .negative_one => .comptime_int_type,
        .undef_usize, .zero_usize, .one_usize => .usize_type,
        .undef_u1, .zero_u1, .one_u1 => .u1_type,
        .zero_u8, .one_u8, .four_u8 => .u8_type,
        .void_value => .void_type,
        .unreachable_value => .noreturn_type,
        .null_value => .null_type,
        .undef_bool, .bool_true, .bool_false => .bool_type,
        .empty_tuple => .empty_tuple_type,

        // This optimization on tags is needed so that indexToKey can call
        // typeOf without being recursive.
        _ => {
            const unwrapped_index = index.unwrap(ip);
            const item = unwrapped_index.getItem(ip);
            return switch (item.tag) {
                .removed => unreachable,

                .type_int_signed,
                .type_int_unsigned,
                .type_array_big,
                .type_array_small,
                .type_vector,
                .type_pointer,
                .type_slice,
                .type_optional,
                .type_anyframe,
                .type_error_union,
                .type_anyerror_union,
                .type_error_set,
                .type_inferred_error_set,
                .type_tuple,
                .type_function,
                .type_struct,
                .type_struct_packed_auto,
                .type_struct_packed_explicit,
                .type_struct_packed_auto_defaults,
                .type_struct_packed_explicit_defaults,
                .type_union,
                .type_union_packed_auto,
                .type_union_packed_explicit,
                .type_enum_auto,
                .type_enum_explicit,
                .type_enum_nonexhaustive,
                .type_opaque,
                => .type_type,

                .undef,
                .opt_null,
                .only_possible_value,
                => @enumFromInt(item.data),

                .simple_type, .simple_value => unreachable, // handled via Index above

                inline .ptr_nav,
                .ptr_comptime_alloc,
                .ptr_uav,
                .ptr_uav_aligned,
                .ptr_comptime_field,
                .ptr_int,
                .ptr_eu_payload,
                .ptr_opt_payload,
                .ptr_elem,
                .ptr_field,
                .ptr_slice,
                .opt_payload,
                .error_union_payload,
                .int_small,
                .error_set_error,
                .error_union_error,
                .enum_tag,
                .variable,
                .threadlocal_variable,
                .@"extern",
                .func_decl,
                .func_instance,
                .func_coerced,
                .union_value,
                .bytes,
                .aggregate,
                .repeated,
                .bitpack,
                => |t| {
                    const extra_list = unwrapped_index.getExtra(ip);
                    return @enumFromInt(extra_list.view().items(.@"0")[item.data + std.meta.fieldIndex(t.Payload(), "ty").?]);
                },

                .int_u8 => .u8_type,
                .int_u16 => .u16_type,
                .int_u32 => .u32_type,
                .int_i32 => .i32_type,
                .int_usize => .usize_type,

                .int_comptime_int_u32,
                .int_comptime_int_i32,
                => .comptime_int_type,

                // Note these are stored in limbs data, not extra data.
                .int_positive,
                .int_negative,
                => {
                    const limbs_list = ip.getLocalShared(unwrapped_index.tid).getLimbs();
                    const int: Int = @bitCast(limbs_list.view().items(.@"0")[item.data..][0..Int.limbs_items_len].*);
                    return int.ty;
                },

                .enum_literal => .enum_literal_type,
                .float_f16 => .f16_type,
                .float_f32 => .f32_type,
                .float_f64 => .f64_type,
                .float_f80 => .f80_type,
                .float_f128 => .f128_type,

                .float_c_longdouble_f80,
                .float_c_longdouble_f128,
                => .c_longdouble_type,

                .float_comptime_float => .comptime_float_type,

                .memoized_call => unreachable,
            };
        },

        .none => unreachable,
    };
}

/// Assumes that the enum's field indexes equal its value tags.
pub fn toEnum(ip: *const InternPool, comptime E: type, i: Index) E {
    const int = ip.indexToKey(i).enum_tag.int;
    return @enumFromInt(ip.indexToKey(int).int.storage.u64);
}

pub fn toFunc(ip: *const InternPool, i: Index) Key.Func {
    return ip.indexToKey(i).func;
}

pub fn aggregateTypeLen(ip: *const InternPool, ty: Index) u64 {
    return switch (ip.indexToKey(ty)) {
        .struct_type => ip.loadStructType(ty).field_types.len,
        .tuple_type => |tuple_type| tuple_type.types.len,
        .array_type => |array_type| array_type.len,
        .vector_type => |vector_type| vector_type.len,
        else => unreachable,
    };
}

pub fn aggregateTypeLenIncludingSentinel(ip: *const InternPool, ty: Index) u64 {
    return switch (ip.indexToKey(ty)) {
        .struct_type => ip.loadStructType(ty).field_types.len,
        .tuple_type => |tuple_type| tuple_type.types.len,
        .array_type => |array_type| array_type.lenIncludingSentinel(),
        .vector_type => |vector_type| vector_type.len,
        else => unreachable,
    };
}

pub fn funcTypeReturnType(ip: *const InternPool, ty: Index) Index {
    const unwrapped_ty = ty.unwrap(ip);
    const ty_extra = unwrapped_ty.getExtra(ip);
    const ty_item = unwrapped_ty.getItem(ip);
    const child_extra, const child_item = switch (ty_item.tag) {
        .type_pointer => child: {
            const child_index: Index = @enumFromInt(ty_extra.view().items(.@"0")[
                ty_item.data + std.meta.fieldIndex(Tag.TypePointer, "child").?
            ]);
            const unwrapped_child = child_index.unwrap(ip);
            break :child .{ unwrapped_child.getExtra(ip), unwrapped_child.getItem(ip) };
        },
        .type_function => .{ ty_extra, ty_item },
        else => unreachable,
    };
    assert(child_item.tag == .type_function);
    return @enumFromInt(child_extra.view().items(.@"0")[
        child_item.data + std.meta.fieldIndex(Tag.TypeFunction, "return_type").?
    ]);
}

pub fn isUndef(ip: *const InternPool, val: Index) bool {
    return val == .undef or val.unwrap(ip).getTag(ip) == .undef;
}

pub fn isVariable(ip: *const InternPool, val: Index) bool {
    return val.unwrap(ip).getTag(ip) == .variable;
}

pub fn getBackingAddrTag(ip: *const InternPool, val: Index) ?Key.Ptr.BaseAddr.Tag {
    var base = val;
    while (true) {
        const unwrapped_base = base.unwrap(ip);
        const base_item = unwrapped_base.getItem(ip);
        switch (base_item.tag) {
            .ptr_nav => return .nav,
            .ptr_comptime_alloc => return .comptime_alloc,
            .ptr_uav,
            .ptr_uav_aligned,
            => return .uav,
            .ptr_comptime_field => return .comptime_field,
            .ptr_int => return .int,
            inline .ptr_eu_payload,
            .ptr_opt_payload,
            .ptr_elem,
            .ptr_field,
            => |tag| base = @enumFromInt(unwrapped_base.getExtra(ip).view().items(.@"0")[
                base_item.data + std.meta.fieldIndex(tag.Payload(), "base").?
            ]),
            inline .ptr_slice => |tag| base = @enumFromInt(unwrapped_base.getExtra(ip).view().items(.@"0")[
                base_item.data + std.meta.fieldIndex(tag.Payload(), "ptr").?
            ]),
            else => return null,
        }
    }
}

/// This is a particularly hot function, so we operate directly on encodings
/// rather than the more straightforward implementation of calling `indexToKey`.
/// Asserts `index` is not `.generic_poison_type`.
pub fn zigTypeTag(ip: *const InternPool, index: Index) std.builtin.TypeId {
    return switch (index) {
        .u0_type,
        .i0_type,
        .u1_type,
        .u8_type,
        .i8_type,
        .u16_type,
        .i16_type,
        .u29_type,
        .u32_type,
        .i32_type,
        .u64_type,
        .i64_type,
        .u80_type,
        .u128_type,
        .i128_type,
        .u256_type,
        .usize_type,
        .isize_type,
        .c_char_type,
        .c_short_type,
        .c_ushort_type,
        .c_int_type,
        .c_uint_type,
        .c_long_type,
        .c_ulong_type,
        .c_longlong_type,
        .c_ulonglong_type,
        => .int,

        .c_longdouble_type,
        .f16_type,
        .f32_type,
        .f64_type,
        .f80_type,
        .f128_type,
        => .float,

        .anyopaque_type => .@"opaque",
        .bool_type => .bool,
        .void_type => .void,
        .type_type => .type,
        .anyerror_type, .adhoc_inferred_error_set_type => .error_set,
        .comptime_int_type => .comptime_int,
        .comptime_float_type => .comptime_float,
        .noreturn_type => .noreturn,
        .anyframe_type => .@"anyframe",
        .null_type => .null,
        .undefined_type => .undefined,
        .enum_literal_type => .enum_literal,

        .ptr_usize_type,
        .ptr_const_comptime_int_type,
        .manyptr_u8_type,
        .manyptr_const_u8_type,
        .manyptr_const_u8_sentinel_0_type,
        .manyptr_const_slice_const_u8_type,
        .slice_const_u8_type,
        .slice_const_u8_sentinel_0_type,
        .slice_const_slice_const_u8_type,
        .manyptr_const_type_type,
        .slice_const_type_type,
        => .pointer,

        .vector_8_i8_type,
        .vector_16_i8_type,
        .vector_32_i8_type,
        .vector_64_i8_type,
        .vector_1_u8_type,
        .vector_2_u8_type,
        .vector_4_u8_type,
        .vector_8_u8_type,
        .vector_16_u8_type,
        .vector_32_u8_type,
        .vector_64_u8_type,
        .vector_2_i16_type,
        .vector_4_i16_type,
        .vector_8_i16_type,
        .vector_16_i16_type,
        .vector_32_i16_type,
        .vector_4_u16_type,
        .vector_8_u16_type,
        .vector_16_u16_type,
        .vector_32_u16_type,
        .vector_2_i32_type,
        .vector_4_i32_type,
        .vector_8_i32_type,
        .vector_16_i32_type,
        .vector_4_u32_type,
        .vector_8_u32_type,
        .vector_16_u32_type,
        .vector_2_i64_type,
        .vector_4_i64_type,
        .vector_8_i64_type,
        .vector_2_u64_type,
        .vector_4_u64_type,
        .vector_8_u64_type,
        .vector_1_u128_type,
        .vector_2_u128_type,
        .vector_1_u256_type,
        .vector_4_f16_type,
        .vector_8_f16_type,
        .vector_16_f16_type,
        .vector_32_f16_type,
        .vector_2_f32_type,
        .vector_4_f32_type,
        .vector_8_f32_type,
        .vector_16_f32_type,
        .vector_2_f64_type,
        .vector_4_f64_type,
        .vector_8_f64_type,
        => .vector,

        .optional_type_type => .optional,
        .optional_noreturn_type => .optional,
        .anyerror_void_error_union_type => .error_union,
        .empty_tuple_type => .@"struct",

        .generic_poison_type => unreachable,

        // values, not types
        .undef => unreachable,
        .undef_bool => unreachable,
        .undef_usize => unreachable,
        .undef_u1 => unreachable,
        .zero => unreachable,
        .zero_usize => unreachable,
        .zero_u1 => unreachable,
        .zero_u8 => unreachable,
        .one => unreachable,
        .one_usize => unreachable,
        .one_u1 => unreachable,
        .one_u8 => unreachable,
        .four_u8 => unreachable,
        .negative_one => unreachable,
        .void_value => unreachable,
        .unreachable_value => unreachable,
        .null_value => unreachable,
        .bool_true => unreachable,
        .bool_false => unreachable,
        .empty_tuple => unreachable,

        _ => switch (index.unwrap(ip).getTag(ip)) {
            .removed => unreachable,

            .type_int_signed,
            .type_int_unsigned,
            => .int,

            .type_array_big,
            .type_array_small,
            => .array,

            .type_vector => .vector,

            .type_pointer,
            .type_slice,
            => .pointer,

            .type_optional => .optional,
            .type_anyframe => .@"anyframe",

            .type_error_union,
            .type_anyerror_union,
            => .error_union,

            .type_error_set,
            .type_inferred_error_set,
            => .error_set,

            .simple_type => unreachable, // handled via Index tag above

            .type_tuple => .@"struct",

            .type_struct,
            .type_struct_packed_auto,
            .type_struct_packed_explicit,
            .type_struct_packed_auto_defaults,
            .type_struct_packed_explicit_defaults,
            => .@"struct",
            .type_union,
            .type_union_packed_auto,
            .type_union_packed_explicit,
            => .@"union",
            .type_enum_auto,
            .type_enum_explicit,
            .type_enum_nonexhaustive,
            => .@"enum",
            .type_opaque,
            => .@"opaque",

            .type_function => .@"fn",

            // values, not types
            .undef,
            .simple_value,
            .ptr_nav,
            .ptr_comptime_alloc,
            .ptr_uav,
            .ptr_uav_aligned,
            .ptr_comptime_field,
            .ptr_int,
            .ptr_eu_payload,
            .ptr_opt_payload,
            .ptr_elem,
            .ptr_field,
            .ptr_slice,
            .opt_payload,
            .opt_null,
            .int_u8,
            .int_u16,
            .int_u32,
            .int_i32,
            .int_usize,
            .int_comptime_int_u32,
            .int_comptime_int_i32,
            .int_small,
            .int_positive,
            .int_negative,
            .error_set_error,
            .error_union_error,
            .error_union_payload,
            .enum_literal,
            .enum_tag,
            .float_f16,
            .float_f32,
            .float_f64,
            .float_f80,
            .float_f128,
            .float_c_longdouble_f80,
            .float_c_longdouble_f128,
            .float_comptime_float,
            .variable,
            .threadlocal_variable,
            .@"extern",
            .func_decl,
            .func_instance,
            .func_coerced,
            .only_possible_value,
            .union_value,
            .bytes,
            .aggregate,
            .repeated,
            .bitpack,
            // memoization, not types
            .memoized_call,
            => unreachable,
        },
        .none => unreachable, // special tag
    };
}

pub fn isFuncBody(ip: *const InternPool, func: Index) bool {
    return switch (func.unwrap(ip).getTag(ip)) {
        .func_decl, .func_instance, .func_coerced => true,
        else => false,
    };
}

fn funcAnalysisPtr(ip: *const InternPool, func: Index) *FuncAnalysis {
    const unwrapped_func = func.unwrap(ip);
    const extra = unwrapped_func.getExtra(ip);
    const item = unwrapped_func.getItem(ip);
    const extra_index = switch (item.tag) {
        .func_decl => item.data + std.meta.fieldIndex(Tag.FuncDecl, "analysis").?,
        .func_instance => item.data + std.meta.fieldIndex(Tag.FuncInstance, "analysis").?,
        .func_coerced => {
            const extra_index = item.data + std.meta.fieldIndex(Tag.FuncCoerced, "func").?;
            const coerced_func_index: Index = @enumFromInt(extra.view().items(.@"0")[extra_index]);
            const unwrapped_coerced_func = coerced_func_index.unwrap(ip);
            const coerced_func_item = unwrapped_coerced_func.getItem(ip);
            return @ptrCast(&unwrapped_coerced_func.getExtra(ip).view().items(.@"0")[
                switch (coerced_func_item.tag) {
                    .func_decl => coerced_func_item.data + std.meta.fieldIndex(Tag.FuncDecl, "analysis").?,
                    .func_instance => coerced_func_item.data + std.meta.fieldIndex(Tag.FuncInstance, "analysis").?,
                    else => unreachable,
                }
            ]);
        },
        else => unreachable,
    };
    return @ptrCast(&extra.view().items(.@"0")[extra_index]);
}

pub fn funcAnalysisUnordered(ip: *const InternPool, func: Index) FuncAnalysis {
    return @atomicLoad(FuncAnalysis, ip.funcAnalysisPtr(func), .unordered);
}

pub fn funcSetHasErrorTrace(ip: *InternPool, io: Io, func: Index, has_error_trace: bool) void {
    const unwrapped_func = func.unwrap(ip);
    const extra_mutex = &ip.getLocal(unwrapped_func.tid).mutate.extra.mutex;
    extra_mutex.lockUncancelable(io);
    defer extra_mutex.unlock(io);

    const analysis_ptr = ip.funcAnalysisPtr(func);
    var analysis = analysis_ptr.*;
    analysis.has_error_trace = has_error_trace;
    @atomicStore(FuncAnalysis, analysis_ptr, analysis, .release);
}

pub fn funcSetDisableInstrumentation(ip: *InternPool, io: Io, func: Index) void {
    const unwrapped_func = func.unwrap(ip);
    const extra_mutex = &ip.getLocal(unwrapped_func.tid).mutate.extra.mutex;
    extra_mutex.lockUncancelable(io);
    defer extra_mutex.unlock(io);

    const analysis_ptr = ip.funcAnalysisPtr(func);
    var analysis = analysis_ptr.*;
    analysis.disable_instrumentation = true;
    @atomicStore(FuncAnalysis, analysis_ptr, analysis, .release);
}

pub fn funcSetDisableIntrinsics(ip: *InternPool, io: Io, func: Index) void {
    const unwrapped_func = func.unwrap(ip);
    const extra_mutex = &ip.getLocal(unwrapped_func.tid).mutate.extra.mutex;
    extra_mutex.lockUncancelable(io);
    defer extra_mutex.unlock(io);

    const analysis_ptr = ip.funcAnalysisPtr(func);
    var analysis = analysis_ptr.*;
    analysis.disable_intrinsics = true;
    @atomicStore(FuncAnalysis, analysis_ptr, analysis, .release);
}

pub fn funcZirBodyInst(ip: *const InternPool, func: Index) TrackedInst.Index {
    const unwrapped_func = func.unwrap(ip);
    const item = unwrapped_func.getItem(ip);
    const item_extra = unwrapped_func.getExtra(ip);
    const zir_body_inst_field_index = std.meta.fieldIndex(Tag.FuncDecl, "zir_body_inst").?;
    switch (item.tag) {
        .func_decl => return @enumFromInt(item_extra.view().items(.@"0")[item.data + zir_body_inst_field_index]),
        .func_instance => {
            const generic_owner_field_index = std.meta.fieldIndex(Tag.FuncInstance, "generic_owner").?;
            const func_decl_index: Index = @enumFromInt(item_extra.view().items(.@"0")[item.data + generic_owner_field_index]);
            const unwrapped_func_decl = func_decl_index.unwrap(ip);
            const func_decl_item = unwrapped_func_decl.getItem(ip);
            const func_decl_extra = unwrapped_func_decl.getExtra(ip);
            assert(func_decl_item.tag == .func_decl);
            return @enumFromInt(func_decl_extra.view().items(.@"0")[func_decl_item.data + zir_body_inst_field_index]);
        },
        .func_coerced => {
            const uncoerced_func_index: Index = @enumFromInt(item_extra.view().items(.@"0")[
                item.data + std.meta.fieldIndex(Tag.FuncCoerced, "func").?
            ]);
            return ip.funcZirBodyInst(uncoerced_func_index);
        },
        else => unreachable,
    }
}

pub fn iesFuncIndex(ip: *const InternPool, ies_index: Index) Index {
    const item = ies_index.unwrap(ip).getItem(ip);
    assert(item.tag == .type_inferred_error_set);
    const func_index: Index = @enumFromInt(item.data);
    switch (func_index.unwrap(ip).getTag(ip)) {
        .func_decl, .func_instance => {},
        else => unreachable, // assertion failed
    }
    return func_index;
}

/// Returns a mutable pointer to the resolved error set type of an inferred
/// error set function. The returned pointer is invalidated when anything is
/// added to `ip`.
fn funcIesResolvedPtr(ip: *const InternPool, func_index: Index) *Index {
    assert(ip.funcAnalysisUnordered(func_index).inferred_error_set);
    const unwrapped_func = func_index.unwrap(ip);
    const func_extra = unwrapped_func.getExtra(ip);
    const func_item = unwrapped_func.getItem(ip);
    const extra_index = switch (func_item.tag) {
        .func_decl => func_item.data + @typeInfo(Tag.FuncDecl).@"struct".fields.len,
        .func_instance => func_item.data + @typeInfo(Tag.FuncInstance).@"struct".fields.len,
        .func_coerced => {
            const uncoerced_func_index: Index = @enumFromInt(func_extra.view().items(.@"0")[
                func_item.data + std.meta.fieldIndex(Tag.FuncCoerced, "func").?
            ]);
            const unwrapped_uncoerced_func = uncoerced_func_index.unwrap(ip);
            const uncoerced_func_item = unwrapped_uncoerced_func.getItem(ip);
            return @ptrCast(&unwrapped_uncoerced_func.getExtra(ip).view().items(.@"0")[
                switch (uncoerced_func_item.tag) {
                    .func_decl => uncoerced_func_item.data + @typeInfo(Tag.FuncDecl).@"struct".fields.len,
                    .func_instance => uncoerced_func_item.data + @typeInfo(Tag.FuncInstance).@"struct".fields.len,
                    else => unreachable,
                }
            ]);
        },
        else => unreachable,
    };
    return @ptrCast(&func_extra.view().items(.@"0")[extra_index]);
}

pub fn funcIesResolvedUnordered(ip: *const InternPool, index: Index) Index {
    return @atomicLoad(Index, ip.funcIesResolvedPtr(index), .unordered);
}

pub fn funcSetIesResolved(ip: *InternPool, io: Io, index: Index, ies: Index) void {
    const unwrapped_func = index.unwrap(ip);
    const extra_mutex = &ip.getLocal(unwrapped_func.tid).mutate.extra.mutex;
    extra_mutex.lockUncancelable(io);
    defer extra_mutex.unlock(io);

    @atomicStore(Index, ip.funcIesResolvedPtr(index), ies, .release);
}

pub fn funcDeclInfo(ip: *const InternPool, index: Index) Key.Func {
    const unwrapped_index = index.unwrap(ip);
    const item = unwrapped_index.getItem(ip);
    assert(item.tag == .func_decl);
    return extraFuncDecl(unwrapped_index.tid, unwrapped_index.getExtra(ip), item.data);
}

pub fn funcTypeParamsLen(ip: *const InternPool, index: Index) u32 {
    const unwrapped_index = index.unwrap(ip);
    const extra_list = unwrapped_index.getExtra(ip);
    const item = unwrapped_index.getItem(ip);
    assert(item.tag == .type_function);
    return extra_list.view().items(.@"0")[item.data + std.meta.fieldIndex(Tag.TypeFunction, "params_len").?];
}

pub fn unwrapCoercedFunc(ip: *const InternPool, index: Index) Index {
    const unwrapped_index = index.unwrap(ip);
    const item = unwrapped_index.getItem(ip);
    return switch (item.tag) {
        .func_coerced => @enumFromInt(unwrapped_index.getExtra(ip).view().items(.@"0")[
            item.data + std.meta.fieldIndex(Tag.FuncCoerced, "func").?
        ]),
        .func_instance, .func_decl => index,
        else => unreachable,
    };
}

/// Puts `name` into `names_slice` at the next index (that being the current length of `map`).
/// Also inserts the name into `map`. If there is an existing field with this name, its index
/// is returned. Otherwise, `null` is returned.
pub fn addFieldName(
    ip: *InternPool,
    names: NullTerminatedString.Slice,
    map: MapIndex,
    name: NullTerminatedString,
) ?u32 {
    const m = map.get(ip);
    const field_idx = m.count();
    const names_slice = names.get(ip);
    names_slice[field_idx] = name;
    const adapter: NullTerminatedString.Adapter = .{ .strings = names_slice[0..field_idx] };
    const gop = m.getOrPutAssumeCapacityAdapted(name, adapter);
    if (gop.found_existing) return @intCast(gop.index);
    assert(gop.index == field_idx);
    return null;
}

/// Like `addFieldName`, but instead of adding a field name to a struct, union, or enum, adds a
/// field tag value for an enum.
pub fn addFieldTagValue(
    ip: *InternPool,
    values: Index.Slice,
    map: MapIndex,
    value: Index,
) ?u32 {
    const m = map.get(ip);
    const field_idx = m.count();
    const values_slice = values.get(ip);
    values_slice[field_idx] = value;
    const adapter: Index.Adapter = .{ .indexes = values_slice[0..field_idx] };
    const gop = m.getOrPutAssumeCapacityAdapted(value, adapter);
    if (gop.found_existing) return @intCast(gop.index);
    assert(gop.index == field_idx);
    return null;
}

/// Used only by `get` for pointer values, and mainly intended to use `Tag.ptr_uav`
/// encoding instead of `Tag.ptr_uav_aligned` when possible.
fn ptrsHaveSameAlignment(ip: *InternPool, a_ty: Index, a_info: Key.PtrType, b_ty: Index) bool {
    if (a_ty == b_ty) return true;
    const b_info = ip.indexToKey(b_ty).ptr_type;
    return a_info.flags.alignment == b_info.flags.alignment and
        (a_info.child == b_info.child or a_info.flags.alignment != .none);
}

const GlobalErrorSet = struct {
    shared: struct {
        names: Names,
        map: Shard.Map(GlobalErrorSet.Index),
    } align(std.atomic.cache_line),
    mutate: struct {
        names: Local.ListMutate,
        map: struct { mutex: Io.Mutex },
    } align(std.atomic.cache_line),

    const Names = Local.List(struct { NullTerminatedString });

    const empty: GlobalErrorSet = .{
        .shared = .{
            .names = .empty,
            .map = .empty,
        },
        .mutate = .{
            .names = .empty,
            .map = .{ .mutex = .init },
        },
    };

    const Index = enum(Zcu.ErrorInt) {
        none = 0,
        _,
    };

    /// Not thread-safe, may only be called from the main thread.
    pub fn getNamesFromMainThread(ges: *const GlobalErrorSet) []const NullTerminatedString {
        const len = ges.mutate.names.len;
        return if (len > 0) ges.shared.names.view().items(.@"0")[0..len] else &.{};
    }

    fn getErrorValue(
        ges: *GlobalErrorSet,
        gpa: Allocator,
        io: Io,
        arena_state: *std.heap.ArenaAllocator.State,
        name: NullTerminatedString,
    ) Allocator.Error!GlobalErrorSet.Index {
        if (name == .empty) return .none;
        const hash = std.hash.int(@intFromEnum(name));
        var map = ges.shared.map.acquire();
        const Map = @TypeOf(map);
        var map_mask = map.header().mask();
        const names = ges.shared.names.acquire();
        var map_index = hash;
        while (true) : (map_index += 1) {
            map_index &= map_mask;
            const entry = &map.entries[map_index];
            const index = entry.acquire();
            if (index == .none) break;
            if (entry.hash != hash) continue;
            if (names.view().items(.@"0")[@intFromEnum(index) - 1] == name) return index;
        }
        ges.mutate.map.mutex.lockUncancelable(io);
        defer ges.mutate.map.mutex.unlock(io);
        if (map.entries != ges.shared.map.entries) {
            map = ges.shared.map;
            map_mask = map.header().mask();
            map_index = hash;
        }
        while (true) : (map_index += 1) {
            map_index &= map_mask;
            const entry = &map.entries[map_index];
            const index = entry.value;
            if (index == .none) break;
            if (entry.hash != hash) continue;
            if (names.view().items(.@"0")[@intFromEnum(index) - 1] == name) return index;
        }
        const mutable_names: Names.Mutable = .{
            .gpa = gpa,
            .io = io,
            .arena = arena_state,
            .mutate = &ges.mutate.names,
            .list = &ges.shared.names,
        };
        try mutable_names.ensureUnusedCapacity(1);
        const map_header = map.header().*;
        if (ges.mutate.names.len < map_header.capacity * 3 / 5) {
            mutable_names.appendAssumeCapacity(.{name});
            const index: GlobalErrorSet.Index = @enumFromInt(mutable_names.mutate.len);
            const entry = &map.entries[map_index];
            entry.hash = hash;
            entry.release(index);
            return index;
        }
        var arena = arena_state.promote(gpa);
        defer arena_state.* = arena.state;
        const new_map_capacity = map_header.capacity * 2;
        const new_map_buf = try arena.allocator().alignedAlloc(
            u8,
            .fromByteUnits(Map.alignment),
            Map.entries_offset + new_map_capacity * @sizeOf(Map.Entry),
        );
        const new_map: Map = .{ .entries = @ptrCast(new_map_buf[Map.entries_offset..].ptr) };
        new_map.header().* = .{ .capacity = new_map_capacity };
        @memset(new_map.entries[0..new_map_capacity], .{ .value = .none, .hash = undefined });
        const new_map_mask = new_map.header().mask();
        map_index = 0;
        while (map_index < map_header.capacity) : (map_index += 1) {
            const entry = &map.entries[map_index];
            const index = entry.value;
            if (index == .none) continue;
            const item_hash = entry.hash;
            var new_map_index = item_hash;
            while (true) : (new_map_index += 1) {
                new_map_index &= new_map_mask;
                const new_entry = &new_map.entries[new_map_index];
                if (new_entry.value != .none) continue;
                new_entry.* = .{
                    .value = index,
                    .hash = item_hash,
                };
                break;
            }
        }
        map = new_map;
        map_index = hash;
        while (true) : (map_index += 1) {
            map_index &= new_map_mask;
            if (map.entries[map_index].value == .none) break;
        }
        mutable_names.appendAssumeCapacity(.{name});
        const index: GlobalErrorSet.Index = @enumFromInt(mutable_names.mutate.len);
        map.entries[map_index] = .{ .value = index, .hash = hash };
        ges.shared.map.release(new_map);
        return index;
    }

    fn getErrorValueIfExists(
        ges: *const GlobalErrorSet,
        name: NullTerminatedString,
    ) ?GlobalErrorSet.Index {
        if (name == .empty) return .none;
        const hash = std.hash.int(@intFromEnum(name));
        const map = ges.shared.map.acquire();
        const map_mask = map.header().mask();
        const names_items = ges.shared.names.acquire().view().items(.@"0");
        var map_index = hash;
        while (true) : (map_index += 1) {
            map_index &= map_mask;
            const entry = &map.entries[map_index];
            const index = entry.acquire();
            if (index == .none) return null;
            if (entry.hash != hash) continue;
            if (names_items[@intFromEnum(index) - 1] == name) return index;
        }
    }
};

pub fn getErrorValue(
    ip: *InternPool,
    gpa: Allocator,
    io: Io,
    tid: Zcu.PerThread.Id,
    name: NullTerminatedString,
) Allocator.Error!Zcu.ErrorInt {
    return @intFromEnum(try ip.global_error_set.getErrorValue(gpa, io, &ip.getLocal(tid).mutate.arena, name));
}

pub fn getErrorValueIfExists(ip: *const InternPool, name: NullTerminatedString) ?Zcu.ErrorInt {
    return @intFromEnum(ip.global_error_set.getErrorValueIfExists(name) orelse return null);
}

const PackedCallingConvention = packed struct(u18) {
    tag: std.builtin.CallingConvention.Tag,
    /// May be ignored depending on `tag`.
    incoming_stack_alignment: Alignment,
    /// Interpretation depends on `tag`.
    extra: u4,

    fn pack(cc: std.builtin.CallingConvention) PackedCallingConvention {
        return switch (cc) {
            inline else => |pl, tag| switch (@TypeOf(pl)) {
                void => .{
                    .tag = tag,
                    .incoming_stack_alignment = .none, // unused
                    .extra = 0, // unused
                },
                std.builtin.CallingConvention.CommonOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = 0, // unused
                },
                std.builtin.CallingConvention.X86RegparmOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = pl.register_params,
                },
                std.builtin.CallingConvention.ArcInterruptOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = @intFromEnum(pl.type),
                },
                std.builtin.CallingConvention.ArmInterruptOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = @intFromEnum(pl.type),
                },
                std.builtin.CallingConvention.MicroblazeInterruptOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = @intFromEnum(pl.type),
                },
                std.builtin.CallingConvention.MipsInterruptOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = @intFromEnum(pl.mode),
                },
                std.builtin.CallingConvention.RiscvInterruptOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = @intFromEnum(pl.mode),
                },
                std.builtin.CallingConvention.ShInterruptOptions => .{
                    .tag = tag,
                    .incoming_stack_alignment = .fromByteUnits(pl.incoming_stack_alignment orelse 0),
                    .extra = @intFromEnum(pl.save),
                },
                else => comptime unreachable,
            },
        };
    }

    fn unpack(cc: PackedCallingConvention) std.builtin.CallingConvention {
        return switch (cc.tag) {
            inline else => |tag| @unionInit(
                std.builtin.CallingConvention,
                @tagName(tag),
                switch (@FieldType(std.builtin.CallingConvention, @tagName(tag))) {
                    void => {},
                    std.builtin.CallingConvention.CommonOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                    },
                    std.builtin.CallingConvention.X86RegparmOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .register_params = @intCast(cc.extra),
                    },
                    std.builtin.CallingConvention.ArcInterruptOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .type = @enumFromInt(cc.extra),
                    },
                    std.builtin.CallingConvention.ArmInterruptOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .type = @enumFromInt(cc.extra),
                    },
                    std.builtin.CallingConvention.MicroblazeInterruptOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .type = @enumFromInt(cc.extra),
                    },
                    std.builtin.CallingConvention.MipsInterruptOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .mode = @enumFromInt(cc.extra),
                    },
                    std.builtin.CallingConvention.RiscvInterruptOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .mode = @enumFromInt(cc.extra),
                    },
                    std.builtin.CallingConvention.ShInterruptOptions => .{
                        .incoming_stack_alignment = cc.incoming_stack_alignment.toByteUnits(),
                        .save = @enumFromInt(cc.extra),
                    },
                    else => comptime unreachable,
                },
            ),
        };
    }
};

/// Asserts that `struct_type` is a non-packed struct type.
/// As well as calling this function, the caller must also populate these arrays:
/// * `field_types`
/// * `field_aligns`
/// * `field_runtime_order`
/// * `field_offsets`
pub fn resolveStructLayout(
    ip: *InternPool,
    io: Io,
    struct_type: Index,
    size: u32,
    alignment: Alignment,
    class: TypeClass,
) void {
    const unwrapped_index = struct_type.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    assert(item.tag == .type_struct);

    extra_items[item.data + std.meta.fieldIndex(Tag.TypeStruct, "size").?] = size;
    const flags: *Tag.TypeStruct.Flags = @ptrCast(&extra_items[item.data + std.meta.fieldIndex(Tag.TypeStruct, "flags").?]);
    flags.class = class;
    flags.alignment = alignment;
}

/// Asserts that `union_type` is a non-packed union type.
/// As well as calling this function, the caller must also populate these arrays:
/// * `field_types`
/// * `field_aligns`
pub fn resolveUnionLayout(
    ip: *InternPool,
    io: Io,
    union_type: Index,
    enum_tag_type: Index,
    class: TypeClass,
    has_runtime_tag: bool,
    size: u32,
    padding: u32,
    alignment: Alignment,
) void {
    const unwrapped_index = union_type.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    assert(item.tag == .type_union);

    extra_items[item.data + std.meta.fieldIndex(Tag.TypeUnion, "enum_tag_type").?] = @intFromEnum(enum_tag_type);
    extra_items[item.data + std.meta.fieldIndex(Tag.TypeUnion, "size").?] = size;
    extra_items[item.data + std.meta.fieldIndex(Tag.TypeUnion, "padding").?] = padding;
    const flags: *Tag.TypeUnion.Flags = @ptrCast(&extra_items[item.data + std.meta.fieldIndex(Tag.TypeUnion, "flags").?]);
    flags.class = class;
    flags.has_runtime_tag = has_runtime_tag;
    flags.alignment = alignment;
}

/// Asserts that `struct_type` is a packed struct type.
pub fn resolvePackedStructLayout(
    ip: *InternPool,
    io: Io,
    struct_type: Index,
    backing_int_type: Index,
) void {
    const unwrapped_index = struct_type.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    switch (item.tag) {
        .type_struct_packed_auto,
        .type_struct_packed_explicit,
        .type_struct_packed_auto_defaults,
        .type_struct_packed_explicit_defaults,
        => {},
        else => unreachable,
    }

    extra_items[item.data + std.meta.fieldIndex(Tag.TypeStructPacked, "backing_int_type").?] = @intFromEnum(backing_int_type);
}

/// Asserts that `union_type` is a packed union type.
pub fn resolvePackedUnionLayout(
    ip: *InternPool,
    io: Io,
    union_type: Index,
    enum_tag_type: Index,
    backing_int_type: Index,
) void {
    const unwrapped_index = union_type.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    switch (item.tag) {
        .type_union_packed_auto,
        .type_union_packed_explicit,
        => {},
        else => unreachable,
    }

    extra_items[item.data + std.meta.fieldIndex(Tag.TypeUnionPacked, "enum_tag_type").?] = @intFromEnum(enum_tag_type);
    extra_items[item.data + std.meta.fieldIndex(Tag.TypeUnionPacked, "backing_int_type").?] = @intFromEnum(backing_int_type);
}

/// Asserts that `enum_type` is an enum type.
pub fn resolveEnumLayout(
    ip: *InternPool,
    io: Io,
    enum_type: Index,
    int_tag_type: Index,
) void {
    const unwrapped_index = enum_type.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    switch (item.tag) {
        .type_enum_auto,
        .type_enum_explicit,
        .type_enum_nonexhaustive,
        => {},
        else => unreachable,
    }

    extra_items[item.data + std.meta.fieldIndex(Tag.TypeEnum, "int_tag_type").?] = @intFromEnum(int_tag_type);
}

/// Sets the "want_layout" flag on the given struct, union, or enum type. Returns true if the flag
/// was *not* already set, meaning we have just discovered the first reference to this type's
/// layout. This flag is never reset to false, and exists purely as an optimization; for details,
/// see doc comments in `LoadedStructType`.
pub fn setWantTypeLayout(ip: *InternPool, io: Io, container_type: Index) bool {
    const unwrapped_index = container_type.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const extra_items = local.shared.extra.view().items(.@"0");
    const item = unwrapped_index.getItem(ip);
    switch (item.tag) {
        .type_struct_packed_auto,
        .type_struct_packed_explicit,
        .type_struct_packed_auto_defaults,
        .type_struct_packed_explicit_defaults,
        => {
            const bits: *Tag.TypeStructPacked.Bits = @ptrCast(&extra_items[
                item.data + std.meta.fieldIndex(Tag.TypeStructPacked, "bits").?
            ]);
            if (bits.want_layout) {
                return false;
            } else {
                bits.want_layout = true;
                return true;
            }
        },

        .type_struct => {
            const flags: *Tag.TypeStruct.Flags = @ptrCast(&extra_items[
                item.data + std.meta.fieldIndex(Tag.TypeStruct, "flags").?
            ]);
            if (flags.want_layout) {
                return false;
            } else {
                flags.want_layout = true;
                return true;
            }
        },

        .type_union_packed_auto,
        .type_union_packed_explicit,
        => {
            const bits: *Tag.TypeUnionPacked.Bits = @ptrCast(&extra_items[
                item.data + std.meta.fieldIndex(Tag.TypeUnionPacked, "bits").?
            ]);
            if (bits.want_layout) {
                return false;
            } else {
                bits.want_layout = true;
                return true;
            }
        },

        .type_union => {
            const flags: *Tag.TypeUnion.Flags = @ptrCast(&extra_items[
                item.data + std.meta.fieldIndex(Tag.TypeUnion, "flags").?
            ]);
            if (flags.want_layout) {
                return false;
            } else {
                flags.want_layout = true;
                return true;
            }
        },

        .type_enum_auto,
        .type_enum_explicit,
        .type_enum_nonexhaustive,
        => {
            const bits: *Tag.TypeEnum.Bits = @ptrCast(&extra_items[
                item.data + std.meta.fieldIndex(Tag.TypeEnum, "bits").?
            ]);
            if (bits.want_layout) {
                return false;
            } else {
                bits.want_layout = true;
                return true;
            }
        },

        else => unreachable,
    }
}

/// Like `setWantTypeLayout`, but for runtime analysis of a function body, using the
/// `FuncAnalysis.want_runtime_analysis` flag.
pub fn setWantRuntimeFnAnalysis(ip: *InternPool, io: Io, func_index: Index) bool {
    const unwrapped_index = func_index.unwrap(ip);

    const local = ip.getLocal(unwrapped_index.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const a = funcAnalysisPtr(ip, func_index);
    if (a.want_runtime_analysis) {
        return false;
    } else {
        a.want_runtime_analysis = true;
        return true;
    }
}

/// Like `setWantTypeLayout`, but for runtime analysis of a `Nav`, using the `Nav.analysis.wanted` flag.
pub fn setWantNavAnalysis(ip: *InternPool, io: Io, nav_index: Nav.Index) bool {
    const unwrapped = nav_index.unwrap(ip);

    const local = ip.getLocal(unwrapped.tid);
    local.mutate.extra.mutex.lockUncancelable(io);
    defer local.mutate.extra.mutex.unlock(io);

    const navs = local.shared.navs.view();

    if (navs.items(.analysis_namespace)[unwrapped.index] == .none) {
        return false;
    }

    const bits = &navs.items(.bits)[unwrapped.index];
    if (bits.want_analysis) {
        return false;
    } else {
        bits.want_analysis = true;
        return true;
    }
}