Add a handle_cycle_error query modifier.

This modifier indicates that a query has a custom handler for cycles.
That custom handler must be found at
`rustc_query_impl::handle_cycle_error::$name`.

This eliminates the need for `specialize_query_vtables`, which is the
current hack to install custom handlers. It's more lines of code in
total, but indicating special treatment of a query via a modifier in
`queries.rs` is more consistent with how other aspects of queries are
handled.

compiler/rustc_macros/src/query.rs

@@ -142,6 +142,7 @@ struct QueryModifiers {
     desc: Desc,
     eval_always: Option<Ident>,
     feedable: Option<Ident>,
+    handle_cycle_error: Option<Ident>,
     no_force: Option<Ident>,
     no_hash: Option<Ident>,
     separate_provide_extern: Option<Ident>,
@@ -156,6 +157,7 @@ fn parse_query_modifiers(input: ParseStream<'_>) -> Result<QueryModifiers> {
     let mut desc = None;
     let mut eval_always = None;
     let mut feedable = None;
+    let mut handle_cycle_error = None;
     let mut no_force = None;
     let mut no_hash = None;
     let mut separate_provide_extern = None;
@@ -190,6 +192,8 @@ macro_rules! try_insert {
             try_insert!(eval_always = modifier);
         } else if modifier == "feedable" {
             try_insert!(feedable = modifier);
+        } else if modifier == "handle_cycle_error" {
+            try_insert!(handle_cycle_error = modifier);
         } else if modifier == "no_force" {
             try_insert!(no_force = modifier);
         } else if modifier == "no_hash" {
@@ -211,6 +215,7 @@ macro_rules! try_insert {
         desc,
         eval_always,
         feedable,
+        handle_cycle_error,
         no_force,
         no_hash,
         separate_provide_extern,
@@ -246,6 +251,7 @@ fn make_modifiers_stream(query: &Query) -> proc_macro2::TokenStream {
         desc,
         eval_always,
         feedable,
+        handle_cycle_error,
         no_force,
         no_hash,
         separate_provide_extern,
@@ -270,6 +276,7 @@ fn make_modifiers_stream(query: &Query) -> proc_macro2::TokenStream {
     };
     let eval_always = eval_always.is_some();
     let feedable = feedable.is_some();
+    let handle_cycle_error = handle_cycle_error.is_some();
     let no_force = no_force.is_some();
     let no_hash = no_hash.is_some();
     let returns_error_guaranteed = returns_error_guaranteed(&query.return_ty);
@@ -290,6 +297,7 @@ fn make_modifiers_stream(query: &Query) -> proc_macro2::TokenStream {
         desc: #desc,
         eval_always: #eval_always,
         feedable: #feedable,
+        handle_cycle_error: #handle_cycle_error,
         no_force: #no_force,
         no_hash: #no_hash,
         returns_error_guaranteed: #returns_error_guaranteed,
@@ -358,6 +366,7 @@ macro_rules! doc_link {
         // `desc` is handled above
         eval_always,
         feedable,
+        handle_cycle_error,
         no_force,
         no_hash,
         separate_provide_extern,

compiler/rustc_middle/src/queries.rs

@@ -583,6 +583,7 @@
     // messages about cycles that then abort.)
     query check_representability(key: LocalDefId) {
         desc { "checking if `{}` is representable", tcx.def_path_str(key) }
+        handle_cycle_error
         // We don't want recursive representability calls to be forced with
         // incremental compilation because, if a cycle occurs, we need the
         // entire cycle to be in memory for diagnostics.
@@ -593,6 +594,7 @@
     /// details, particularly on the modifiers.
     query check_representability_adt_ty(key: Ty<'tcx>) {
         desc { "checking if `{}` is representable", key }
+        handle_cycle_error
         no_force
     }
 
@@ -1032,6 +1034,7 @@
     query variances_of(def_id: DefId) -> &'tcx [ty::Variance] {
         desc { "computing the variances of `{}`", tcx.def_path_str(def_id) }
         cache_on_disk
+        handle_cycle_error
         separate_provide_extern
     }
 
@@ -1164,6 +1167,7 @@
     query fn_sig(key: DefId) -> ty::EarlyBinder<'tcx, ty::PolyFnSig<'tcx>> {
         desc { "computing function signature of `{}`", tcx.def_path_str(key) }
         cache_on_disk
+        handle_cycle_error
         separate_provide_extern
     }
 
@@ -1756,6 +1760,7 @@
     ) -> Result<ty::layout::TyAndLayout<'tcx>, &'tcx ty::layout::LayoutError<'tcx>> {
         depth_limit
         desc { "computing layout of `{}`", key.value }
+        handle_cycle_error
     }
 
     /// Compute a `FnAbi` suitable for indirect calls, i.e. to `fn` pointers.

compiler/rustc_middle/src/query/modifiers.rs

@@ -58,6 +58,14 @@
 /// Generate a `feed` method to set the query's value from another query.
 pub(crate) struct feedable;
 
+/// # `handle_cycle_error` query modifier
+///
+/// The default behaviour for a query cycle is to emit a cycle error and halt
+/// compilation. Queries with this modifier will instead use a custom handler,
+/// which must be provided at `rustc_query_impl::handle_cycle_error::$name`,
+/// where `$name` is the query name.
+pub(crate) struct handle_cycle_error;
+
 /// # `no_force` query modifier
 ///
 /// Dep nodes of queries with this modifier will never be "forced" when trying

compiler/rustc_middle/src/query/plumbing.rs

@@ -304,6 +304,7 @@ fn $name:ident($($K:tt)*) -> $V:ty
                     desc: $desc:expr,
                     eval_always: $eval_always:literal,
                     feedable: $feedable:literal,
+                    handle_cycle_error: $handle_cycle_error:literal,
                     no_force: $no_force:literal,
                     no_hash: $no_hash:literal,
                     returns_error_guaranteed: $returns_error_guaranteed:literal,

compiler/rustc_query_impl/src/dep_kind_vtables.rs

@@ -138,6 +138,7 @@ fn $name:ident($K:ty) -> $V:ty
                     desc: $desc:expr,
                     eval_always: $eval_always:literal,
                     feedable: $feedable:literal,
+                    handle_cycle_error: $handle_cycle_error:literal,
                     no_force: $no_force:literal,
                     no_hash: $no_hash:literal,
                     returns_error_guaranteed: $returns_error_guaranteed:literal,

compiler/rustc_query_impl/src/handle_cycle_error.rs

@@ -9,48 +9,29 @@
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_middle::bug;
-use rustc_middle::queries::{QueryVTables, TaggedQueryKey};
+use rustc_middle::queries::TaggedQueryKey;
 use rustc_middle::query::Cycle;
-use rustc_middle::query::erase::erase_val;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::{ErrorGuaranteed, Span};
 
 use crate::job::create_cycle_error;
 
-pub(crate) fn specialize_query_vtables<'tcx>(vtables: &mut QueryVTables<'tcx>) {
+// Default cycle handler used for all queries that don't use the `handle_cycle_error` query
-    vtables.fn_sig.handle_cycle_error_fn = |tcx, key, _, err| {
+// modifier.
-        let guar = err.delay_as_bug();
-        erase_val(fn_sig(tcx, key, guar))
-    };
-
-    vtables.check_representability.handle_cycle_error_fn =
-        |tcx, _, cycle, _err| check_representability(tcx, cycle);
-
-    vtables.check_representability_adt_ty.handle_cycle_error_fn =
-        |tcx, _, cycle, _err| check_representability(tcx, cycle);
-
-    vtables.variances_of.handle_cycle_error_fn = |tcx, key, _, err| {
-        let _guar = err.delay_as_bug();
-        erase_val(variances_of(tcx, key))
-    };
-
-    vtables.layout_of.handle_cycle_error_fn = |tcx, _, cycle, err| {
-        let _guar = err.delay_as_bug();
-        erase_val(Err(layout_of(tcx, cycle)))
-    }
-}
-
 pub(crate) fn default(err: Diag<'_>) -> ! {
     let guar = err.emit();
     guar.raise_fatal()
 }
 
-fn fn_sig<'tcx>(
+pub(crate) fn fn_sig<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
-    guar: ErrorGuaranteed,
+    _: Cycle<'tcx>,
+    err: Diag<'_>,
 ) -> ty::EarlyBinder<'tcx, ty::PolyFnSig<'tcx>> {
+    let guar = err.delay_as_bug();
+
     let err = Ty::new_error(tcx, guar);
 
     let arity = if let Some(node) = tcx.hir_get_if_local(def_id)
@@ -71,7 +52,25 @@ fn fn_sig<'tcx>(
     )))
 }
 
-fn check_representability<'tcx>(tcx: TyCtxt<'tcx>, cycle: Cycle<'tcx>) -> ! {
+pub(crate) fn check_representability<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    _key: LocalDefId,
+    cycle: Cycle<'tcx>,
+    _err: Diag<'_>,
+) {
+    check_representability_inner(tcx, cycle);
+}
+
+pub(crate) fn check_representability_adt_ty<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    _key: Ty<'tcx>,
+    cycle: Cycle<'tcx>,
+    _err: Diag<'_>,
+) {
+    check_representability_inner(tcx, cycle);
+}
+
+fn check_representability_inner<'tcx>(tcx: TyCtxt<'tcx>, cycle: Cycle<'tcx>) -> ! {
     let mut item_and_field_ids = Vec::new();
     let mut representable_ids = FxHashSet::default();
     for frame in &cycle.frames {
@@ -102,7 +101,13 @@ fn check_representability<'tcx>(tcx: TyCtxt<'tcx>, cycle: Cycle<'tcx>) -> ! {
     guar.raise_fatal()
 }
 
-fn variances_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx [ty::Variance] {
+pub(crate) fn variances_of<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def_id: DefId,
+    _cycle: Cycle<'tcx>,
+    err: Diag<'_>,
+) -> &'tcx [ty::Variance] {
+    let _guar = err.delay_as_bug();
     let n = tcx.generics_of(def_id).own_params.len();
     tcx.arena.alloc_from_iter(iter::repeat_n(ty::Bivariant, n))
 }
@@ -126,7 +131,13 @@ fn search_for_cycle_permutation<Q, T>(
     otherwise()
 }
 
-fn layout_of<'tcx>(tcx: TyCtxt<'tcx>, cycle: Cycle<'tcx>) -> &'tcx ty::layout::LayoutError<'tcx> {
+pub(crate) fn layout_of<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    _key: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
+    cycle: Cycle<'tcx>,
+    err: Diag<'_>,
+) -> Result<ty::layout::TyAndLayout<'tcx>, &'tcx ty::layout::LayoutError<'tcx>> {
+    let _guar = err.delay_as_bug();
     let diag = search_for_cycle_permutation(
         &cycle.frames,
         |frames| {

compiler/rustc_query_impl/src/lib.rs

@@ -48,11 +48,9 @@ pub fn query_system<'tcx>(
     on_disk_cache: Option<OnDiskCache>,
     incremental: bool,
 ) -> QuerySystem<'tcx> {
-    let mut query_vtables = query_impl::make_query_vtables(incremental);
-    handle_cycle_error::specialize_query_vtables(&mut query_vtables);
     QuerySystem {
         arenas: Default::default(),
-        query_vtables,
+        query_vtables: query_impl::make_query_vtables(incremental),
         side_effects: Default::default(),
         on_disk_cache,
         local_providers,

compiler/rustc_query_impl/src/query_impl.rs

@@ -22,6 +22,7 @@ fn $name:ident($K:ty) -> $V:ty
                     desc: $desc:expr,
                     eval_always: $eval_always:literal,
                     feedable: $feedable:literal,
+                    handle_cycle_error: $handle_cycle_error:literal,
                     no_force: $no_force:literal,
                     no_hash: $no_hash:literal,
                     returns_error_guaranteed: $returns_error_guaranteed:literal,
@@ -144,7 +145,6 @@ pub(crate) fn make_query_vtable<'tcx>(incremental: bool)
                     -> QueryVTable<'tcx, rustc_middle::queries::$name::Cache<'tcx>>
                 {
                     use rustc_middle::queries::$name::Value;
-
                     QueryVTable {
                         name: stringify!($name),
                         eval_always: $eval_always,
@@ -177,9 +177,13 @@ pub(crate) fn make_query_vtable<'tcx>(incremental: bool)
                         #[cfg(not($cache_on_disk))]
                         try_load_from_disk_fn: |_tcx, _key, _prev_index, _index| None,
 
-                        // The default just emits `err` and then aborts.
+                        #[cfg($handle_cycle_error)]
-                        // `handle_cycle_error::specialize_query_vtables` overwrites this default
+                        handle_cycle_error_fn: |tcx, key, cycle, err| {
-                        // for certain queries.
+                            use rustc_middle::query::erase::erase_val;
+
+                            erase_val($crate::handle_cycle_error::$name(tcx, key, cycle, err))
+                        },
+                        #[cfg(not($handle_cycle_error))]
                         handle_cycle_error_fn: |_tcx, _key, _cycle, err| {
                             $crate::handle_cycle_error::default(err)
                         },