Auto merge of #94934 - Lireer:const-prop-lint, r=oli-obk

Separate const prop lints from optimizations

r? `@oli-obk`

Separates lints and optimizations during const prop by moving the lints into their own file and checking them during post borrowck cleanup.
Thanks to `@oli-obk` for mentoring me.

compiler/rustc_mir_transform/src/const_prop.rs

@@ -6,30 +6,25 @@
 use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def::DefKind;
-use rustc_hir::HirId;
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
 use rustc_middle::mir::visit::{
     MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
 };
 use rustc_middle::mir::{
-    AssertKind, BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind,
-    Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
-    StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
+    BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
+    Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
+    RETURN_PLACE,
 };
 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
 use rustc_middle::ty::subst::{InternalSubsts, Subst};
-use rustc_middle::ty::{
-    self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeFoldable,
-};
-use rustc_session::lint;
+use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
 use rustc_span::{def_id::DefId, Span};
 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 
 use crate::MirPass;
-use rustc_const_eval::const_eval::ConstEvalErr;
 use rustc_const_eval::interpret::{
     self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame,
     ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, MemoryKind, OpTy,
@@ -318,9 +313,8 @@ struct ConstPropagator<'mir, 'tcx> {
     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
     tcx: TyCtxt<'tcx>,
     param_env: ParamEnv<'tcx>,
-    // FIXME(eddyb) avoid cloning these two fields more than once,
-    // by accessing them through `ecx` instead.
-    source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
+    // FIXME(eddyb) avoid cloning this field more than once,
+    // by accessing it through `ecx` instead.
     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
     // the last known `SourceInfo` here and just keep revisiting it.
@@ -412,9 +406,8 @@ fn new(
             ecx,
             tcx,
             param_env,
-            // FIXME(eddyb) avoid cloning these two fields more than once,
-            // by accessing them through `ecx` instead.
-            source_scopes: body.source_scopes.clone(),
+            // FIXME(eddyb) avoid cloning this field more than once,
+            // by accessing it through `ecx` instead.
             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
             local_decls: body.local_decls.clone(),
             source_info: None,
@@ -445,10 +438,6 @@ fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, lo
             LocalState { value: LocalValue::Unallocated, layout: Cell::new(None) };
     }
 
-    fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
-        source_info.scope.lint_root(&self.source_scopes)
-    }
-
     fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
     where
         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
@@ -471,45 +460,13 @@ fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
     }
 
     /// Returns the value, if any, of evaluating `c`.
-    fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
+    fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
         // FIXME we need to revisit this for #67176
         if c.needs_subst() {
             return None;
         }
 
-        match self.ecx.mir_const_to_op(&c.literal, None) {
-            Ok(op) => Some(op),
-            Err(error) => {
-                let tcx = self.ecx.tcx.at(c.span);
-                let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
-                if let Some(lint_root) = self.lint_root(source_info) {
-                    let lint_only = match c.literal {
-                        ConstantKind::Ty(ct) => match ct.val() {
-                            // Promoteds must lint and not error as the user didn't ask for them
-                            ConstKind::Unevaluated(ty::Unevaluated {
-                                def: _,
-                                substs: _,
-                                promoted: Some(_),
-                            }) => true,
-                            // Out of backwards compatibility we cannot report hard errors in unused
-                            // generic functions using associated constants of the generic parameters.
-                            _ => c.literal.needs_subst(),
-                        },
-                        ConstantKind::Val(_, ty) => ty.needs_subst(),
-                    };
-                    if lint_only {
-                        // Out of backwards compatibility we cannot report hard errors in unused
-                        // generic functions using associated constants of the generic parameters.
-                        err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
-                    } else {
-                        err.report_as_error(tcx, "erroneous constant used");
-                    }
-                } else {
-                    err.report_as_error(tcx, "erroneous constant used");
-                }
-                None
-            }
-        }
+        self.ecx.mir_const_to_op(&c.literal, None).ok()
     }
 
     /// Returns the value, if any, of evaluating `place`.
@@ -520,49 +477,22 @@ fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 
     /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
     /// or `eval_place`, depending on the variant of `Operand` used.
-    fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
+    fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
         match *op {
-            Operand::Constant(ref c) => self.eval_constant(c, source_info),
+            Operand::Constant(ref c) => self.eval_constant(c),
             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
         }
     }
 
-    fn report_assert_as_lint(
-        &self,
-        lint: &'static lint::Lint,
-        source_info: SourceInfo,
-        message: &'static str,
-        panic: AssertKind<impl std::fmt::Debug>,
-    ) {
-        if let Some(lint_root) = self.lint_root(source_info) {
-            self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
-                let mut err = lint.build(message);
-                err.span_label(source_info.span, format!("{:?}", panic));
-                err.emit();
-            });
-        }
-    }
-
-    fn check_unary_op(
-        &mut self,
-        op: UnOp,
-        arg: &Operand<'tcx>,
-        source_info: SourceInfo,
-    ) -> Option<()> {
-        if let (val, true) = self.use_ecx(|this| {
+    fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
+        if self.use_ecx(|this| {
             let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
-            Ok((val, overflow))
+            Ok(overflow)
         })? {
             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
             // appropriate to use.
             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
-            self.report_assert_as_lint(
-                lint::builtin::ARITHMETIC_OVERFLOW,
-                source_info,
-                "this arithmetic operation will overflow",
-                AssertKind::OverflowNeg(val.to_const_int()),
-            );
             return None;
         }
 
@@ -574,7 +504,6 @@ fn check_binary_op(
         op: BinOp,
         left: &Operand<'tcx>,
         right: &Operand<'tcx>,
-        source_info: SourceInfo,
     ) -> Option<()> {
         let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
         let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
@@ -589,25 +518,6 @@ fn check_binary_op(
             let r_bits = r.to_scalar().ok();
             let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
             if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
-                debug!("check_binary_op: reporting assert for {:?}", source_info);
-                self.report_assert_as_lint(
-                    lint::builtin::ARITHMETIC_OVERFLOW,
-                    source_info,
-                    "this arithmetic operation will overflow",
-                    AssertKind::Overflow(
-                        op,
-                        match l {
-                            Some(l) => l.to_const_int(),
-                            // Invent a dummy value, the diagnostic ignores it anyway
-                            None => ConstInt::new(
-                                ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
-                                left_ty.is_signed(),
-                                left_ty.is_ptr_sized_integral(),
-                            ),
-                        },
-                        r.to_const_int(),
-                    ),
-                );
                 return None;
             }
         }
@@ -618,12 +528,6 @@ fn check_binary_op(
                 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
                 Ok(overflow)
             })? {
-                self.report_assert_as_lint(
-                    lint::builtin::ARITHMETIC_OVERFLOW,
-                    source_info,
-                    "this arithmetic operation will overflow",
-                    AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
-                );
                 return None;
             }
         }
@@ -656,12 +560,7 @@ fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
         }
     }
 
-    fn const_prop(
-        &mut self,
-        rvalue: &Rvalue<'tcx>,
-        source_info: SourceInfo,
-        place: Place<'tcx>,
-    ) -> Option<()> {
+    fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
         // Perform any special handling for specific Rvalue types.
         // Generally, checks here fall into one of two categories:
         //   1. Additional checking to provide useful lints to the user
@@ -676,11 +575,11 @@ fn const_prop(
             // lint.
             Rvalue::UnaryOp(op, arg) => {
                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
-                self.check_unary_op(*op, arg, source_info)?;
+                self.check_unary_op(*op, arg)?;
             }
             Rvalue::BinaryOp(op, box (left, right)) => {
                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
-                self.check_binary_op(*op, left, right, source_info)?;
+                self.check_binary_op(*op, left, right)?;
             }
             Rvalue::CheckedBinaryOp(op, box (left, right)) => {
                 trace!(
@@ -689,7 +588,7 @@ fn const_prop(
                     left,
                     right
                 );
-                self.check_binary_op(*op, left, right, source_info)?;
+                self.check_binary_op(*op, left, right)?;
             }
 
             // Do not try creating references (#67862)
@@ -1071,7 +970,7 @@ fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
     fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
         trace!("visit_constant: {:?}", constant);
         self.super_constant(constant, location);
-        self.eval_constant(constant, self.source_info.unwrap());
+        self.eval_constant(constant);
     }
 
     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
@@ -1080,7 +979,7 @@ fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Locatio
         self.source_info = Some(source_info);
         if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
             let can_const_prop = self.ecx.machine.can_const_prop[place.local];
-            if let Some(()) = self.const_prop(rval, source_info, place) {
+            if let Some(()) = self.const_prop(rval, place) {
                 // This will return None if the above `const_prop` invocation only "wrote" a
                 // type whose creation requires no write. E.g. a generator whose initial state
                 // consists solely of uninitialized memory (so it doesn't capture any locals).
@@ -1164,57 +1063,12 @@ fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Loca
         self.source_info = Some(source_info);
         self.super_terminator(terminator, location);
         match &mut terminator.kind {
-            TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
-                if let Some(ref value) = self.eval_operand(&cond, source_info) {
+            TerminatorKind::Assert { expected, ref mut cond, .. } => {
+                if let Some(ref value) = self.eval_operand(&cond) {
                     trace!("assertion on {:?} should be {:?}", value, expected);
                     let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
                     let value_const = self.ecx.read_scalar(&value).unwrap();
                     if expected != value_const {
-                        enum DbgVal<T> {
-                            Val(T),
-                            Underscore,
-                        }
-                        impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
-                            fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-                                match self {
-                                    Self::Val(val) => val.fmt(fmt),
-                                    Self::Underscore => fmt.write_str("_"),
-                                }
-                            }
-                        }
-                        let mut eval_to_int = |op| {
-                            // This can be `None` if the lhs wasn't const propagated and we just
-                            // triggered the assert on the value of the rhs.
-                            self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
-                                DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
-                            })
-                        };
-                        let msg = match msg {
-                            AssertKind::DivisionByZero(op) => {
-                                Some(AssertKind::DivisionByZero(eval_to_int(op)))
-                            }
-                            AssertKind::RemainderByZero(op) => {
-                                Some(AssertKind::RemainderByZero(eval_to_int(op)))
-                            }
-                            AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
-                                // Division overflow is *UB* in the MIR, and different than the
-                                // other overflow checks.
-                                Some(AssertKind::Overflow(
-                                    *bin_op,
-                                    eval_to_int(op1),
-                                    eval_to_int(op2),
-                                ))
-                            }
-                            AssertKind::BoundsCheck { ref len, ref index } => {
-                                let len = eval_to_int(len);
-                                let index = eval_to_int(index);
-                                Some(AssertKind::BoundsCheck { len, index })
-                            }
-                            // Remaining overflow errors are already covered by checks on the binary operators.
-                            AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
-                            // Need proper const propagator for these.
-                            _ => None,
-                        };
                         // Poison all places this operand references so that further code
                         // doesn't use the invalid value
                         match cond {
@@ -1223,14 +1077,6 @@ fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                             }
                             Operand::Constant(_) => {}
                         }
-                        if let Some(msg) = msg {
-                            self.report_assert_as_lint(
-                                lint::builtin::UNCONDITIONAL_PANIC,
-                                source_info,
-                                "this operation will panic at runtime",
-                                msg,
-                            );
-                        }
                     } else {
                         if self.should_const_prop(value) {
                             if let ScalarMaybeUninit::Scalar(scalar) = value_const {

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -0,0 +1,1035 @@
+//! Propagates constants for early reporting of statically known
+//! assertion failures
+
+use std::cell::Cell;
+
+use rustc_ast::Mutability;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_hir::def::DefKind;
+use rustc_hir::HirId;
+use rustc_index::bit_set::BitSet;
+use rustc_index::vec::IndexVec;
+use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
+use rustc_middle::mir::{
+    AssertKind, BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind,
+    Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
+    StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
+};
+use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
+use rustc_middle::ty::subst::{InternalSubsts, Subst};
+use rustc_middle::ty::{
+    self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeFoldable,
+};
+use rustc_session::lint;
+use rustc_span::{def_id::DefId, Span};
+use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
+use rustc_target::spec::abi::Abi;
+use rustc_trait_selection::traits;
+
+use crate::MirLint;
+use rustc_const_eval::const_eval::ConstEvalErr;
+use rustc_const_eval::interpret::{
+    self, compile_time_machine, AllocId, ConstAllocation, Frame, ImmTy, InterpCx, InterpResult,
+    LocalState, LocalValue, MemPlace, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy, Scalar,
+    ScalarMaybeUninit, StackPopCleanup, StackPopUnwind,
+};
+
+/// The maximum number of bytes that we'll allocate space for a local or the return value.
+/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
+/// Severely regress performance.
+const MAX_ALLOC_LIMIT: u64 = 1024;
+
+/// Macro for machine-specific `InterpError` without allocation.
+/// (These will never be shown to the user, but they help diagnose ICEs.)
+macro_rules! throw_machine_stop_str {
+    ($($tt:tt)*) => {{
+        // We make a new local type for it. The type itself does not carry any information,
+        // but its vtable (for the `MachineStopType` trait) does.
+        struct Zst;
+        // Printing this type shows the desired string.
+        impl std::fmt::Display for Zst {
+            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                write!(f, $($tt)*)
+            }
+        }
+        impl rustc_middle::mir::interpret::MachineStopType for Zst {}
+        throw_machine_stop!(Zst)
+    }};
+}
+
+pub struct ConstProp;
+
+impl<'tcx> MirLint<'tcx> for ConstProp {
+    fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
+        // will be evaluated by miri and produce its errors there
+        if body.source.promoted.is_some() {
+            return;
+        }
+
+        let def_id = body.source.def_id().expect_local();
+        let is_fn_like = tcx.hir().get_by_def_id(def_id).fn_kind().is_some();
+        let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
+
+        // Only run const prop on functions, methods, closures and associated constants
+        if !is_fn_like && !is_assoc_const {
+            // skip anon_const/statics/consts because they'll be evaluated by miri anyway
+            trace!("ConstProp skipped for {:?}", def_id);
+            return;
+        }
+
+        let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
+        // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
+        // computing their layout.
+        if is_generator {
+            trace!("ConstProp skipped for generator {:?}", def_id);
+            return;
+        }
+
+        // Check if it's even possible to satisfy the 'where' clauses
+        // for this item.
+        // This branch will never be taken for any normal function.
+        // However, it's possible to `#!feature(trivial_bounds)]` to write
+        // a function with impossible to satisfy clauses, e.g.:
+        // `fn foo() where String: Copy {}`
+        //
+        // We don't usually need to worry about this kind of case,
+        // since we would get a compilation error if the user tried
+        // to call it. However, since we can do const propagation
+        // even without any calls to the function, we need to make
+        // sure that it even makes sense to try to evaluate the body.
+        // If there are unsatisfiable where clauses, then all bets are
+        // off, and we just give up.
+        //
+        // We manually filter the predicates, skipping anything that's not
+        // "global". We are in a potentially generic context
+        // (e.g. we are evaluating a function without substituting generic
+        // parameters, so this filtering serves two purposes:
+        //
+        // 1. We skip evaluating any predicates that we would
+        // never be able prove are unsatisfiable (e.g. `<T as Foo>`
+        // 2. We avoid trying to normalize predicates involving generic
+        // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
+        // the normalization code (leading to cycle errors), since
+        // it's usually never invoked in this way.
+        let predicates = tcx
+            .predicates_of(def_id.to_def_id())
+            .predicates
+            .iter()
+            .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
+        if traits::impossible_predicates(
+            tcx,
+            traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
+        ) {
+            trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
+            return;
+        }
+
+        trace!("ConstProp starting for {:?}", def_id);
+
+        let dummy_body = &Body::new(
+            body.source,
+            body.basic_blocks().clone(),
+            body.source_scopes.clone(),
+            body.local_decls.clone(),
+            Default::default(),
+            body.arg_count,
+            Default::default(),
+            body.span,
+            body.generator_kind(),
+            body.tainted_by_errors,
+        );
+
+        // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
+        // constants, instead of just checking for const-folding succeeding.
+        // That would require a uniform one-def no-mutation analysis
+        // and RPO (or recursing when needing the value of a local).
+        let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
+        optimization_finder.visit_body(body);
+
+        trace!("ConstProp done for {:?}", def_id);
+    }
+}
+
+struct ConstPropMachine<'mir, 'tcx> {
+    /// The virtual call stack.
+    stack: Vec<Frame<'mir, 'tcx>>,
+    /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
+    written_only_inside_own_block_locals: FxHashSet<Local>,
+    /// Locals that need to be cleared after every block terminates.
+    only_propagate_inside_block_locals: BitSet<Local>,
+    can_const_prop: IndexVec<Local, ConstPropMode>,
+}
+
+impl ConstPropMachine<'_, '_> {
+    fn new(
+        only_propagate_inside_block_locals: BitSet<Local>,
+        can_const_prop: IndexVec<Local, ConstPropMode>,
+    ) -> Self {
+        Self {
+            stack: Vec::new(),
+            written_only_inside_own_block_locals: Default::default(),
+            only_propagate_inside_block_locals,
+            can_const_prop,
+        }
+    }
+}
+
+impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
+    compile_time_machine!(<'mir, 'tcx>);
+    const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
+
+    type MemoryKind = !;
+
+    type MemoryExtra = ();
+
+    fn load_mir(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::InstanceDef<'tcx>,
+    ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
+        throw_machine_stop_str!("calling functions isn't supported in ConstProp")
+    }
+
+    fn find_mir_or_eval_fn(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::Instance<'tcx>,
+        _abi: Abi,
+        _args: &[OpTy<'tcx>],
+        _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
+        _unwind: StackPopUnwind,
+    ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
+        Ok(None)
+    }
+
+    fn call_intrinsic(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::Instance<'tcx>,
+        _args: &[OpTy<'tcx>],
+        _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
+        _unwind: StackPopUnwind,
+    ) -> InterpResult<'tcx> {
+        throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
+    }
+
+    fn assert_panic(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _msg: &rustc_middle::mir::AssertMessage<'tcx>,
+        _unwind: Option<rustc_middle::mir::BasicBlock>,
+    ) -> InterpResult<'tcx> {
+        bug!("panics terminators are not evaluated in ConstProp")
+    }
+
+    fn binary_ptr_op(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        _bin_op: BinOp,
+        _left: &ImmTy<'tcx>,
+        _right: &ImmTy<'tcx>,
+    ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
+        // We can't do this because aliasing of memory can differ between const eval and llvm
+        throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
+    }
+
+    fn access_local(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
+        local: Local,
+    ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
+        let l = &frame.locals[local];
+
+        if l.value == LocalValue::Unallocated {
+            throw_machine_stop_str!("tried to access an uninitialized local")
+        }
+
+        l.access()
+    }
+
+    fn access_local_mut<'a>(
+        ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
+        frame: usize,
+        local: Local,
+    ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
+    {
+        if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
+            throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
+        }
+        if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
+            trace!(
+                "mutating local {:?} which is restricted to its block. \
+                Will remove it from const-prop after block is finished.",
+                local
+            );
+            ecx.machine.written_only_inside_own_block_locals.insert(local);
+        }
+        ecx.machine.stack[frame].locals[local].access_mut()
+    }
+
+    fn before_access_global(
+        _memory_extra: &(),
+        _alloc_id: AllocId,
+        alloc: ConstAllocation<'tcx, Self::PointerTag, Self::AllocExtra>,
+        _static_def_id: Option<DefId>,
+        is_write: bool,
+    ) -> InterpResult<'tcx> {
+        if is_write {
+            throw_machine_stop_str!("can't write to global");
+        }
+        // If the static allocation is mutable, then we can't const prop it as its content
+        // might be different at runtime.
+        if alloc.inner().mutability == Mutability::Mut {
+            throw_machine_stop_str!("can't access mutable globals in ConstProp");
+        }
+
+        Ok(())
+    }
+
+    #[inline(always)]
+    fn init_frame_extra(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        frame: Frame<'mir, 'tcx>,
+    ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
+        Ok(frame)
+    }
+
+    #[inline(always)]
+    fn stack<'a>(
+        ecx: &'a InterpCx<'mir, 'tcx, Self>,
+    ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
+        &ecx.machine.stack
+    }
+
+    #[inline(always)]
+    fn stack_mut<'a>(
+        ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
+    ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
+        &mut ecx.machine.stack
+    }
+}
+
+/// Finds optimization opportunities on the MIR.
+struct ConstPropagator<'mir, 'tcx> {
+    ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    // FIXME(eddyb) avoid cloning these two fields more than once,
+    // by accessing them through `ecx` instead.
+    source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
+    local_decls: IndexVec<Local, LocalDecl<'tcx>>,
+    // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
+    // the last known `SourceInfo` here and just keep revisiting it.
+    source_info: Option<SourceInfo>,
+}
+
+impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
+    type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
+
+    #[inline]
+    fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
+        err
+    }
+}
+
+impl HasDataLayout for ConstPropagator<'_, '_> {
+    #[inline]
+    fn data_layout(&self) -> &TargetDataLayout {
+        &self.tcx.data_layout
+    }
+}
+
+impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
+    #[inline]
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+}
+
+impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
+    #[inline]
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+}
+
+impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
+    fn new(
+        body: &Body<'tcx>,
+        dummy_body: &'mir Body<'tcx>,
+        tcx: TyCtxt<'tcx>,
+    ) -> ConstPropagator<'mir, 'tcx> {
+        let def_id = body.source.def_id();
+        let substs = &InternalSubsts::identity_for_item(tcx, def_id);
+        let param_env = tcx.param_env_reveal_all_normalized(def_id);
+
+        let span = tcx.def_span(def_id);
+        // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
+        // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
+        // `layout_of` query invocations.
+        let can_const_prop = CanConstProp::check(tcx, param_env, body);
+        let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
+        for (l, mode) in can_const_prop.iter_enumerated() {
+            if *mode == ConstPropMode::OnlyInsideOwnBlock {
+                only_propagate_inside_block_locals.insert(l);
+            }
+        }
+        let mut ecx = InterpCx::new(
+            tcx,
+            span,
+            param_env,
+            ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
+            (),
+        );
+
+        let ret = ecx
+            .layout_of(body.return_ty().subst(tcx, substs))
+            .ok()
+            // Don't bother allocating memory for ZST types which have no values
+            // or for large values.
+            .filter(|ret_layout| {
+                !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
+            })
+            .map(|ret_layout| {
+                ecx.allocate(ret_layout, MemoryKind::Stack)
+                    .expect("couldn't perform small allocation")
+                    .into()
+            });
+
+        ecx.push_stack_frame(
+            Instance::new(def_id, substs),
+            dummy_body,
+            ret.as_ref(),
+            StackPopCleanup::Root { cleanup: false },
+        )
+        .expect("failed to push initial stack frame");
+
+        ConstPropagator {
+            ecx,
+            tcx,
+            param_env,
+            // FIXME(eddyb) avoid cloning these two fields more than once,
+            // by accessing them through `ecx` instead.
+            source_scopes: body.source_scopes.clone(),
+            //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
+            local_decls: body.local_decls.clone(),
+            source_info: None,
+        }
+    }
+
+    fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
+        let op = match self.ecx.eval_place_to_op(place, None) {
+            Ok(op) => op,
+            Err(e) => {
+                trace!("get_const failed: {}", e);
+                return None;
+            }
+        };
+
+        // Try to read the local as an immediate so that if it is representable as a scalar, we can
+        // handle it as such, but otherwise, just return the value as is.
+        Some(match self.ecx.try_read_immediate(&op) {
+            Ok(Ok(imm)) => imm.into(),
+            _ => op,
+        })
+    }
+
+    /// Remove `local` from the pool of `Locals`. Allows writing to them,
+    /// but not reading from them anymore.
+    fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
+        ecx.frame_mut().locals[local] =
+            LocalState { value: LocalValue::Unallocated, layout: Cell::new(None) };
+    }
+
+    fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
+        source_info.scope.lint_root(&self.source_scopes)
+    }
+
+    fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
+    where
+        F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
+    {
+        match f(self) {
+            Ok(val) => Some(val),
+            Err(error) => {
+                trace!("InterpCx operation failed: {:?}", error);
+                // Some errors shouldn't come up because creating them causes
+                // an allocation, which we should avoid. When that happens,
+                // dedicated error variants should be introduced instead.
+                assert!(
+                    !error.kind().formatted_string(),
+                    "const-prop encountered formatting error: {}",
+                    error
+                );
+                None
+            }
+        }
+    }
+
+    /// Returns the value, if any, of evaluating `c`.
+    fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
+        // FIXME we need to revisit this for #67176
+        if c.needs_subst() {
+            return None;
+        }
+
+        match self.ecx.mir_const_to_op(&c.literal, None) {
+            Ok(op) => Some(op),
+            Err(error) => {
+                let tcx = self.ecx.tcx.at(c.span);
+                let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
+                if let Some(lint_root) = self.lint_root(source_info) {
+                    let lint_only = match c.literal {
+                        ConstantKind::Ty(ct) => match ct.val() {
+                            // Promoteds must lint and not error as the user didn't ask for them
+                            ConstKind::Unevaluated(ty::Unevaluated {
+                                def: _,
+                                substs: _,
+                                promoted: Some(_),
+                            }) => true,
+                            // Out of backwards compatibility we cannot report hard errors in unused
+                            // generic functions using associated constants of the generic parameters.
+                            _ => c.literal.needs_subst(),
+                        },
+                        ConstantKind::Val(_, ty) => ty.needs_subst(),
+                    };
+                    if lint_only {
+                        // Out of backwards compatibility we cannot report hard errors in unused
+                        // generic functions using associated constants of the generic parameters.
+                        err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
+                    } else {
+                        err.report_as_error(tcx, "erroneous constant used");
+                    }
+                } else {
+                    err.report_as_error(tcx, "erroneous constant used");
+                }
+                None
+            }
+        }
+    }
+
+    /// Returns the value, if any, of evaluating `place`.
+    fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
+        trace!("eval_place(place={:?})", place);
+        self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
+    }
+
+    /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
+    /// or `eval_place`, depending on the variant of `Operand` used.
+    fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
+        match *op {
+            Operand::Constant(ref c) => self.eval_constant(c, source_info),
+            Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
+        }
+    }
+
+    fn report_assert_as_lint(
+        &self,
+        lint: &'static lint::Lint,
+        source_info: SourceInfo,
+        message: &'static str,
+        panic: AssertKind<impl std::fmt::Debug>,
+    ) {
+        if let Some(lint_root) = self.lint_root(source_info) {
+            self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
+                let mut err = lint.build(message);
+                err.span_label(source_info.span, format!("{:?}", panic));
+                err.emit();
+            });
+        }
+    }
+
+    fn check_unary_op(
+        &mut self,
+        op: UnOp,
+        arg: &Operand<'tcx>,
+        source_info: SourceInfo,
+    ) -> Option<()> {
+        if let (val, true) = self.use_ecx(|this| {
+            let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
+            let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
+            Ok((val, overflow))
+        })? {
+            // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
+            // appropriate to use.
+            assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
+            self.report_assert_as_lint(
+                lint::builtin::ARITHMETIC_OVERFLOW,
+                source_info,
+                "this arithmetic operation will overflow",
+                AssertKind::OverflowNeg(val.to_const_int()),
+            );
+            return None;
+        }
+
+        Some(())
+    }
+
+    fn check_binary_op(
+        &mut self,
+        op: BinOp,
+        left: &Operand<'tcx>,
+        right: &Operand<'tcx>,
+        source_info: SourceInfo,
+    ) -> Option<()> {
+        let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
+        let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
+        // Check for exceeding shifts *even if* we cannot evaluate the LHS.
+        if op == BinOp::Shr || op == BinOp::Shl {
+            let r = r?;
+            // We need the type of the LHS. We cannot use `place_layout` as that is the type
+            // of the result, which for checked binops is not the same!
+            let left_ty = left.ty(&self.local_decls, self.tcx);
+            let left_size = self.ecx.layout_of(left_ty).ok()?.size;
+            let right_size = r.layout.size;
+            let r_bits = r.to_scalar().ok();
+            let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
+            if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
+                debug!("check_binary_op: reporting assert for {:?}", source_info);
+                self.report_assert_as_lint(
+                    lint::builtin::ARITHMETIC_OVERFLOW,
+                    source_info,
+                    "this arithmetic operation will overflow",
+                    AssertKind::Overflow(
+                        op,
+                        match l {
+                            Some(l) => l.to_const_int(),
+                            // Invent a dummy value, the diagnostic ignores it anyway
+                            None => ConstInt::new(
+                                ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
+                                left_ty.is_signed(),
+                                left_ty.is_ptr_sized_integral(),
+                            ),
+                        },
+                        r.to_const_int(),
+                    ),
+                );
+                return None;
+            }
+        }
+
+        if let (Some(l), Some(r)) = (&l, &r) {
+            // The remaining operators are handled through `overflowing_binary_op`.
+            if self.use_ecx(|this| {
+                let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
+                Ok(overflow)
+            })? {
+                self.report_assert_as_lint(
+                    lint::builtin::ARITHMETIC_OVERFLOW,
+                    source_info,
+                    "this arithmetic operation will overflow",
+                    AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
+                );
+                return None;
+            }
+        }
+        Some(())
+    }
+
+    fn const_prop(
+        &mut self,
+        rvalue: &Rvalue<'tcx>,
+        source_info: SourceInfo,
+        place: Place<'tcx>,
+    ) -> Option<()> {
+        // Perform any special handling for specific Rvalue types.
+        // Generally, checks here fall into one of two categories:
+        //   1. Additional checking to provide useful lints to the user
+        //        - In this case, we will do some validation and then fall through to the
+        //          end of the function which evals the assignment.
+        //   2. Working around bugs in other parts of the compiler
+        //        - In this case, we'll return `None` from this function to stop evaluation.
+        match rvalue {
+            // Additional checking: give lints to the user if an overflow would occur.
+            // We do this here and not in the `Assert` terminator as that terminator is
+            // only sometimes emitted (overflow checks can be disabled), but we want to always
+            // lint.
+            Rvalue::UnaryOp(op, arg) => {
+                trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
+                self.check_unary_op(*op, arg, source_info)?;
+            }
+            Rvalue::BinaryOp(op, box (left, right)) => {
+                trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
+                self.check_binary_op(*op, left, right, source_info)?;
+            }
+            Rvalue::CheckedBinaryOp(op, box (left, right)) => {
+                trace!(
+                    "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
+                    op,
+                    left,
+                    right
+                );
+                self.check_binary_op(*op, left, right, source_info)?;
+            }
+
+            // Do not try creating references (#67862)
+            Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
+                trace!("skipping AddressOf | Ref for {:?}", place);
+
+                // This may be creating mutable references or immutable references to cells.
+                // If that happens, the pointed to value could be mutated via that reference.
+                // Since we aren't tracking references, the const propagator loses track of what
+                // value the local has right now.
+                // Thus, all locals that have their reference taken
+                // must not take part in propagation.
+                Self::remove_const(&mut self.ecx, place.local);
+
+                return None;
+            }
+            Rvalue::ThreadLocalRef(def_id) => {
+                trace!("skipping ThreadLocalRef({:?})", def_id);
+
+                return None;
+            }
+
+            // There's no other checking to do at this time.
+            Rvalue::Aggregate(..)
+            | Rvalue::Use(..)
+            | Rvalue::Repeat(..)
+            | Rvalue::Len(..)
+            | Rvalue::Cast(..)
+            | Rvalue::ShallowInitBox(..)
+            | Rvalue::Discriminant(..)
+            | Rvalue::NullaryOp(..) => {}
+        }
+
+        // FIXME we need to revisit this for #67176
+        if rvalue.needs_subst() {
+            return None;
+        }
+
+        self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
+    }
+}
+
+/// The mode that `ConstProp` is allowed to run in for a given `Local`.
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum ConstPropMode {
+    /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
+    FullConstProp,
+    /// The `Local` can only be propagated into and from its own block.
+    OnlyInsideOwnBlock,
+    /// The `Local` can be propagated into but reads cannot be propagated.
+    OnlyPropagateInto,
+    /// The `Local` cannot be part of propagation at all. Any statement
+    /// referencing it either for reading or writing will not get propagated.
+    NoPropagation,
+}
+
+struct CanConstProp {
+    can_const_prop: IndexVec<Local, ConstPropMode>,
+    // False at the beginning. Once set, no more assignments are allowed to that local.
+    found_assignment: BitSet<Local>,
+    // Cache of locals' information
+    local_kinds: IndexVec<Local, LocalKind>,
+}
+
+impl CanConstProp {
+    /// Returns true if `local` can be propagated
+    fn check<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        body: &Body<'tcx>,
+    ) -> IndexVec<Local, ConstPropMode> {
+        let mut cpv = CanConstProp {
+            can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
+            found_assignment: BitSet::new_empty(body.local_decls.len()),
+            local_kinds: IndexVec::from_fn_n(
+                |local| body.local_kind(local),
+                body.local_decls.len(),
+            ),
+        };
+        for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
+            let ty = body.local_decls[local].ty;
+            match tcx.layout_of(param_env.and(ty)) {
+                Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
+                // Either the layout fails to compute, then we can't use this local anyway
+                // or the local is too large, then we don't want to.
+                _ => {
+                    *val = ConstPropMode::NoPropagation;
+                    continue;
+                }
+            }
+            // Cannot use args at all
+            // Cannot use locals because if x < y { y - x } else { x - y } would
+            //        lint for x != y
+            // FIXME(oli-obk): lint variables until they are used in a condition
+            // FIXME(oli-obk): lint if return value is constant
+            if cpv.local_kinds[local] == LocalKind::Arg {
+                *val = ConstPropMode::OnlyPropagateInto;
+                trace!(
+                    "local {:?} can't be const propagated because it's a function argument",
+                    local
+                );
+            } else if cpv.local_kinds[local] == LocalKind::Var {
+                *val = ConstPropMode::OnlyInsideOwnBlock;
+                trace!(
+                    "local {:?} will only be propagated inside its block, because it's a user variable",
+                    local
+                );
+            }
+        }
+        cpv.visit_body(&body);
+        cpv.can_const_prop
+    }
+}
+
+impl Visitor<'_> for CanConstProp {
+    fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
+        use rustc_middle::mir::visit::PlaceContext::*;
+        match context {
+            // Projections are fine, because `&mut foo.x` will be caught by
+            // `MutatingUseContext::Borrow` elsewhere.
+            MutatingUse(MutatingUseContext::Projection)
+            // These are just stores, where the storing is not propagatable, but there may be later
+            // mutations of the same local via `Store`
+            | MutatingUse(MutatingUseContext::Call)
+            | MutatingUse(MutatingUseContext::AsmOutput)
+            // Actual store that can possibly even propagate a value
+            | MutatingUse(MutatingUseContext::Store) => {
+                if !self.found_assignment.insert(local) {
+                    match &mut self.can_const_prop[local] {
+                        // If the local can only get propagated in its own block, then we don't have
+                        // to worry about multiple assignments, as we'll nuke the const state at the
+                        // end of the block anyway, and inside the block we overwrite previous
+                        // states as applicable.
+                        ConstPropMode::OnlyInsideOwnBlock => {}
+                        ConstPropMode::NoPropagation => {}
+                        ConstPropMode::OnlyPropagateInto => {}
+                        other @ ConstPropMode::FullConstProp => {
+                            trace!(
+                                "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
+                                local, other,
+                            );
+                            *other = ConstPropMode::OnlyInsideOwnBlock;
+                        }
+                    }
+                }
+            }
+            // Reading constants is allowed an arbitrary number of times
+            NonMutatingUse(NonMutatingUseContext::Copy)
+            | NonMutatingUse(NonMutatingUseContext::Move)
+            | NonMutatingUse(NonMutatingUseContext::Inspect)
+            | NonMutatingUse(NonMutatingUseContext::Projection)
+            | NonUse(_) => {}
+
+            // These could be propagated with a smarter analysis or just some careful thinking about
+            // whether they'd be fine right now.
+            MutatingUse(MutatingUseContext::Yield)
+            | MutatingUse(MutatingUseContext::Drop)
+            | MutatingUse(MutatingUseContext::Retag)
+            // These can't ever be propagated under any scheme, as we can't reason about indirect
+            // mutation.
+            | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
+            | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
+            | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
+            | NonMutatingUse(NonMutatingUseContext::AddressOf)
+            | MutatingUse(MutatingUseContext::Borrow)
+            | MutatingUse(MutatingUseContext::AddressOf) => {
+                trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
+                self.can_const_prop[local] = ConstPropMode::NoPropagation;
+            }
+        }
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
+    fn visit_body(&mut self, body: &Body<'tcx>) {
+        for (bb, data) in body.basic_blocks().iter_enumerated() {
+            self.visit_basic_block_data(bb, data);
+        }
+    }
+
+    fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
+        self.super_operand(operand, location);
+    }
+
+    fn visit_constant(&mut self, constant: &Constant<'tcx>, location: Location) {
+        trace!("visit_constant: {:?}", constant);
+        self.super_constant(constant, location);
+        self.eval_constant(constant, self.source_info.unwrap());
+    }
+
+    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
+        trace!("visit_statement: {:?}", statement);
+        let source_info = statement.source_info;
+        self.source_info = Some(source_info);
+        if let StatementKind::Assign(box (place, ref rval)) = statement.kind {
+            let can_const_prop = self.ecx.machine.can_const_prop[place.local];
+            if let Some(()) = self.const_prop(rval, source_info, place) {
+                match can_const_prop {
+                    ConstPropMode::OnlyInsideOwnBlock => {
+                        trace!(
+                            "found local restricted to its block. \
+                                Will remove it from const-prop after block is finished. Local: {:?}",
+                            place.local
+                        );
+                    }
+                    ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
+                        trace!("can't propagate into {:?}", place);
+                        if place.local != RETURN_PLACE {
+                            Self::remove_const(&mut self.ecx, place.local);
+                        }
+                    }
+                    ConstPropMode::FullConstProp => {}
+                }
+            } else {
+                // Const prop failed, so erase the destination, ensuring that whatever happens
+                // from here on, does not know about the previous value.
+                // This is important in case we have
+                // ```rust
+                // let mut x = 42;
+                // x = SOME_MUTABLE_STATIC;
+                // // x must now be uninit
+                // ```
+                // FIXME: we overzealously erase the entire local, because that's easier to
+                // implement.
+                trace!(
+                    "propagation into {:?} failed.
+                        Nuking the entire site from orbit, it's the only way to be sure",
+                    place,
+                );
+                Self::remove_const(&mut self.ecx, place.local);
+            }
+        } else {
+            match statement.kind {
+                StatementKind::SetDiscriminant { ref place, .. } => {
+                    match self.ecx.machine.can_const_prop[place.local] {
+                        ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
+                            if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
+                                trace!("propped discriminant into {:?}", place);
+                            } else {
+                                Self::remove_const(&mut self.ecx, place.local);
+                            }
+                        }
+                        ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
+                            Self::remove_const(&mut self.ecx, place.local);
+                        }
+                    }
+                }
+                StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
+                    let frame = self.ecx.frame_mut();
+                    frame.locals[local].value =
+                        if let StatementKind::StorageLive(_) = statement.kind {
+                            LocalValue::Unallocated
+                        } else {
+                            LocalValue::Dead
+                        };
+                }
+                _ => {}
+            }
+        }
+
+        self.super_statement(statement, location);
+    }
+
+    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
+        let source_info = terminator.source_info;
+        self.source_info = Some(source_info);
+        self.super_terminator(terminator, location);
+        match &terminator.kind {
+            TerminatorKind::Assert { expected, ref msg, ref cond, .. } => {
+                if let Some(ref value) = self.eval_operand(&cond, source_info) {
+                    trace!("assertion on {:?} should be {:?}", value, expected);
+                    let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
+                    let value_const = self.ecx.read_scalar(&value).unwrap();
+                    if expected != value_const {
+                        enum DbgVal<T> {
+                            Val(T),
+                            Underscore,
+                        }
+                        impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
+                            fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                                match self {
+                                    Self::Val(val) => val.fmt(fmt),
+                                    Self::Underscore => fmt.write_str("_"),
+                                }
+                            }
+                        }
+                        let mut eval_to_int = |op| {
+                            // This can be `None` if the lhs wasn't const propagated and we just
+                            // triggered the assert on the value of the rhs.
+                            self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
+                                DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
+                            })
+                        };
+                        let msg = match msg {
+                            AssertKind::DivisionByZero(op) => {
+                                Some(AssertKind::DivisionByZero(eval_to_int(op)))
+                            }
+                            AssertKind::RemainderByZero(op) => {
+                                Some(AssertKind::RemainderByZero(eval_to_int(op)))
+                            }
+                            AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
+                                // Division overflow is *UB* in the MIR, and different than the
+                                // other overflow checks.
+                                Some(AssertKind::Overflow(
+                                    *bin_op,
+                                    eval_to_int(op1),
+                                    eval_to_int(op2),
+                                ))
+                            }
+                            AssertKind::BoundsCheck { ref len, ref index } => {
+                                let len = eval_to_int(len);
+                                let index = eval_to_int(index);
+                                Some(AssertKind::BoundsCheck { len, index })
+                            }
+                            // Remaining overflow errors are already covered by checks on the binary operators.
+                            AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
+                            // Need proper const propagator for these.
+                            _ => None,
+                        };
+                        // Poison all places this operand references so that further code
+                        // doesn't use the invalid value
+                        match cond {
+                            Operand::Move(ref place) | Operand::Copy(ref place) => {
+                                Self::remove_const(&mut self.ecx, place.local);
+                            }
+                            Operand::Constant(_) => {}
+                        }
+                        if let Some(msg) = msg {
+                            self.report_assert_as_lint(
+                                lint::builtin::UNCONDITIONAL_PANIC,
+                                source_info,
+                                "this operation will panic at runtime",
+                                msg,
+                            );
+                        }
+                    }
+                }
+            }
+            // None of these have Operands to const-propagate.
+            TerminatorKind::Goto { .. }
+            | TerminatorKind::Resume
+            | TerminatorKind::Abort
+            | TerminatorKind::Return
+            | TerminatorKind::Unreachable
+            | TerminatorKind::Drop { .. }
+            | TerminatorKind::DropAndReplace { .. }
+            | TerminatorKind::Yield { .. }
+            | TerminatorKind::GeneratorDrop
+            | TerminatorKind::FalseEdge { .. }
+            | TerminatorKind::FalseUnwind { .. }
+            | TerminatorKind::SwitchInt { .. }
+            | TerminatorKind::Call { .. }
+            | TerminatorKind::InlineAsm { .. } => {}
+        }
+
+        // We remove all Locals which are restricted in propagation to their containing blocks and
+        // which were modified in the current block.
+        // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
+        let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
+        for &local in locals.iter() {
+            Self::remove_const(&mut self.ecx, local);
+        }
+        locals.clear();
+        // Put it back so we reuse the heap of the storage
+        self.ecx.machine.written_only_inside_own_block_locals = locals;
+        if cfg!(debug_assertions) {
+            // Ensure we are correctly erasing locals with the non-debug-assert logic.
+            for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
+                assert!(
+                    self.get_const(local.into()).is_none()
+                        || self
+                            .layout_of(self.local_decls[local].ty)
+                            .map_or(true, |layout| layout.is_zst())
+                )
+            }
+        }
+    }
+}

compiler/rustc_mir_transform/src/lib.rs

@@ -49,6 +49,7 @@
 mod const_debuginfo;
 mod const_goto;
 mod const_prop;
+mod const_prop_lint;
 mod coverage;
 mod deaggregator;
 mod deduplicate_blocks;
@@ -430,6 +431,7 @@ fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tc
         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
         // and it can help optimizations.
         &deaggregator::Deaggregator,
+        &Lint(const_prop_lint::ConstProp),
     ];
 
     pm::run_passes(tcx, body, post_borrowck_cleanup);

src/test/codegen/consts.rs

@@ -10,7 +10,7 @@
 // CHECK: @STATIC = {{.*}}, align 4
 
 // This checks the constants from inline_enum_const
-// CHECK: @alloc9 = {{.*}}, align 2
+// CHECK: @alloc12 = {{.*}}, align 2
 
 // This checks the constants from {low,high}_align_const, they share the same
 // constant, but the alignment differs, so the higher one should be used

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-impl.rs

@@ -4,12 +4,12 @@ trait Foo {
     const BAR: u32;
 }
 
-const IMPL_REF_BAR: u32 = GlobalImplRef::BAR; //~ ERROR E0391
+const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
 
 struct GlobalImplRef;
 
 impl GlobalImplRef {
-    const BAR: u32 = IMPL_REF_BAR;
+    const BAR: u32 = IMPL_REF_BAR; //~ ERROR E0391
 }
 
 fn main() {}

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-impl.stderr

@@ -1,9 +1,15 @@
-error[E0391]: cycle detected when simplifying constant for the type system `IMPL_REF_BAR`
+error[E0391]: cycle detected when elaborating drops for `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
+   |
+LL |     const BAR: u32 = IMPL_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which requires normalizing `IMPL_REF_BAR`...
+note: ...which requires simplifying constant for the type system `IMPL_REF_BAR`...
   --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
    |
 LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
 note: ...which requires simplifying constant for the type system `IMPL_REF_BAR`...
   --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
    |
@@ -35,8 +41,7 @@ note: ...which requires caching mir of `<impl at $DIR/issue-24949-assoc-const-st
    |
 LL |     const BAR: u32 = IMPL_REF_BAR;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: ...which requires normalizing `IMPL_REF_BAR`...
-   = note: ...which again requires simplifying constant for the type system `IMPL_REF_BAR`, completing the cycle
+   = note: ...which again requires elaborating drops for `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`, completing the cycle
    = note: cycle used when running analysis passes on this crate
 
 error: aborting due to previous error

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-trait-default.rs

@@ -5,10 +5,10 @@ trait Foo {
 }
 
 trait FooDefault {
-    const BAR: u32 = DEFAULT_REF_BAR;
+    const BAR: u32 = DEFAULT_REF_BAR; //~ ERROR E0391
 }
 
-const DEFAULT_REF_BAR: u32 = <GlobalDefaultRef>::BAR; //~ ERROR E0391
+const DEFAULT_REF_BAR: u32 = <GlobalDefaultRef>::BAR;
 
 struct GlobalDefaultRef;
 

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-trait-default.stderr

@@ -1,9 +1,15 @@
-error[E0391]: cycle detected when simplifying constant for the type system `DEFAULT_REF_BAR`
+error[E0391]: cycle detected when elaborating drops for `FooDefault::BAR`
+  --> $DIR/issue-24949-assoc-const-static-recursion-trait-default.rs:8:5
+   |
+LL |     const BAR: u32 = DEFAULT_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which requires normalizing `DEFAULT_REF_BAR`...
+note: ...which requires simplifying constant for the type system `DEFAULT_REF_BAR`...
   --> $DIR/issue-24949-assoc-const-static-recursion-trait-default.rs:11:1
    |
 LL | const DEFAULT_REF_BAR: u32 = <GlobalDefaultRef>::BAR;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
 note: ...which requires simplifying constant for the type system `DEFAULT_REF_BAR`...
   --> $DIR/issue-24949-assoc-const-static-recursion-trait-default.rs:11:1
    |
@@ -35,8 +41,7 @@ note: ...which requires caching mir of `FooDefault::BAR` for CTFE...
    |
 LL |     const BAR: u32 = DEFAULT_REF_BAR;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: ...which requires normalizing `DEFAULT_REF_BAR`...
-   = note: ...which again requires simplifying constant for the type system `DEFAULT_REF_BAR`, completing the cycle
+   = note: ...which again requires elaborating drops for `FooDefault::BAR`, completing the cycle
    = note: cycle used when running analysis passes on this crate
 
 error: aborting due to previous error

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-trait.rs

@@ -4,12 +4,12 @@ trait Foo {
     const BAR: u32;
 }
 
-const TRAIT_REF_BAR: u32 = <GlobalTraitRef>::BAR; //~ ERROR E0391
+const TRAIT_REF_BAR: u32 = <GlobalTraitRef>::BAR;
 
 struct GlobalTraitRef;
 
 impl Foo for GlobalTraitRef {
-    const BAR: u32 = TRAIT_REF_BAR;
+    const BAR: u32 = TRAIT_REF_BAR; //~ ERROR E0391
 }
 
 fn main() {}

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-trait.stderr

@@ -1,9 +1,15 @@
-error[E0391]: cycle detected when simplifying constant for the type system `TRAIT_REF_BAR`
+error[E0391]: cycle detected when elaborating drops for `<impl at $DIR/issue-24949-assoc-const-static-recursion-trait.rs:11:1: 13:2>::BAR`
+  --> $DIR/issue-24949-assoc-const-static-recursion-trait.rs:12:5
+   |
+LL |     const BAR: u32 = TRAIT_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which requires normalizing `TRAIT_REF_BAR`...
+note: ...which requires simplifying constant for the type system `TRAIT_REF_BAR`...
   --> $DIR/issue-24949-assoc-const-static-recursion-trait.rs:7:1
    |
 LL | const TRAIT_REF_BAR: u32 = <GlobalTraitRef>::BAR;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
 note: ...which requires simplifying constant for the type system `TRAIT_REF_BAR`...
   --> $DIR/issue-24949-assoc-const-static-recursion-trait.rs:7:1
    |
@@ -35,8 +41,7 @@ note: ...which requires caching mir of `<impl at $DIR/issue-24949-assoc-const-st
    |
 LL |     const BAR: u32 = TRAIT_REF_BAR;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: ...which requires normalizing `TRAIT_REF_BAR`...
-   = note: ...which again requires simplifying constant for the type system `TRAIT_REF_BAR`, completing the cycle
+   = note: ...which again requires elaborating drops for `<impl at $DIR/issue-24949-assoc-const-static-recursion-trait.rs:11:1: 13:2>::BAR`, completing the cycle
    = note: cycle used when running analysis passes on this crate
 
 error: aborting due to previous error

src/test/ui/const_prop/inline_spans.rs

@@ -1,18 +1,15 @@
-// build-fail
+// build-pass
 // compile-flags: -Zmir-opt-level=3
+// Overflow can't be detected by const prop
+// could only be detected after optimizations
 
 #![deny(warnings)]
 
 fn main() {
     let _ = add(u8::MAX, 1);
-    //~^ NOTE in this expansion of inlined source
-    //~| NOTE in this expansion of inlined source
 }
 
 #[inline(always)]
 fn add(x: u8, y: u8) -> u8 {
     x + y
-    //~^ ERROR this arithmetic operation will overflow
-    //~| NOTE attempt to compute `u8::MAX + 1_u8`, which would overflow
-    //~| NOTE `#[deny(arithmetic_overflow)]` on by default
 }

src/test/ui/const_prop/inline_spans.stderr

@@ -1,13 +0,0 @@
-error: this arithmetic operation will overflow
-  --> $DIR/inline_spans.rs:14:5
-   |
-LL |     let _ = add(u8::MAX, 1);
-   |             --------------- in this inlined function call
-...
-LL |     x + y
-   |     ^^^^^ attempt to compute `u8::MAX + 1_u8`, which would overflow
-   |
-   = note: `#[deny(arithmetic_overflow)]` on by default
-
-error: aborting due to previous error
-

src/test/ui/consts/const-eval/const-eval-query-stack.stderr

@@ -21,6 +21,7 @@ LL |     let x: &'static i32 = &X;
    |                            ^ referenced constant has errors
 query stack during panic:
 #0 [try_normalize_mir_const_after_erasing_regions] normalizing `main::promoted[1]`
-#1 [optimized_mir] optimizing MIR for `main`
-#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
+#1 [mir_drops_elaborated_and_const_checked] elaborating drops for `main`
+#2 [optimized_mir] optimizing MIR for `main`
+#3 [collect_and_partition_mono_items] collect_and_partition_mono_items
 end of query stack

src/test/ui/consts/const-eval/issue-49296.stderr

@@ -2,7 +2,7 @@ error[E0080]: evaluation of constant value failed
   --> $DIR/issue-49296.rs:9:16
    |
 LL | const X: u64 = *wat(42);
-   |                ^^^^^^^^ pointer to alloc2 was dereferenced after this allocation got freed
+   |                ^^^^^^^^ pointer to alloc3 was dereferenced after this allocation got freed
 
 error: aborting due to previous error
 

src/test/ui/consts/const-eval/panic-assoc-never-type.rs

@@ -13,4 +13,5 @@ impl PrintName {
 
 fn main() {
     let _ = PrintName::VOID;
+    //~^ ERROR erroneous constant used [E0080]
 }

src/test/ui/consts/const-eval/panic-assoc-never-type.stderr

@@ -6,6 +6,12 @@ LL |     const VOID: ! = panic!();
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error
+error[E0080]: erroneous constant used
+  --> $DIR/panic-assoc-never-type.rs:15:13
+   |
+LL |     let _ = PrintName::VOID;
+   |             ^^^^^^^^^^^^^^^ referenced constant has errors
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/promoted_errors.noopt.stderr

@@ -1,3 +1,45 @@
+warning: this arithmetic operation will overflow
+  --> $DIR/promoted_errors.rs:15:5
+   |
+LL |     0 - 1
+   |     ^^^^^ attempt to compute `0_u32 - 1_u32`, which would overflow
+   |
+note: the lint level is defined here
+  --> $DIR/promoted_errors.rs:11:20
+   |
+LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
+   |                    ^^^^^^^^^^^^^^^^^^^
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:21:5
+   |
+LL |     1 / 0
+   |     ^^^^^ attempt to divide `1_i32` by zero
+   |
+note: the lint level is defined here
+  --> $DIR/promoted_errors.rs:11:41
+   |
+LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
+   |                                         ^^^^^^^^^^^^^^^^^^^
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:27:5
+   |
+LL |     1 / (1 - 1)
+   |     ^^^^^^^^^^^ attempt to divide `1_i32` by zero
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:31:5
+   |
+LL |     1 / (false as i32)
+   |     ^^^^^^^^^^^^^^^^^^ attempt to divide `1_i32` by zero
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:35:5
+   |
+LL |     [1, 2, 3][4]
+   |     ^^^^^^^^^^^^ index out of bounds: the length is 3 but the index is 4
+
 warning: any use of this value will cause an error
   --> $DIR/promoted_errors.rs:15:5
    |
@@ -6,7 +48,7 @@ LL |       0 - 1
    |       |
    |       attempt to compute `0_u32 - 1_u32`, which would overflow
    |       inside `overflow` at $DIR/promoted_errors.rs:15:5
-   |       inside `X` at $DIR/promoted_errors.rs:38:29
+   |       inside `X` at $DIR/promoted_errors.rs:43:29
 ...
 LL | / const X: () = {
 LL | |     let _x: &'static u32 = &overflow();
@@ -26,7 +68,7 @@ LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
 warning: any use of this value will cause an error
-  --> $DIR/promoted_errors.rs:38:28
+  --> $DIR/promoted_errors.rs:43:28
    |
 LL | / const X: () = {
 LL | |     let _x: &'static u32 = &overflow();
@@ -41,5 +83,5 @@ LL | | };
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-warning: 2 warnings emitted
+warning: 7 warnings emitted
 

src/test/ui/consts/const-eval/promoted_errors.opt.stderr

@@ -1,12 +1,54 @@
+warning: this arithmetic operation will overflow
+  --> $DIR/promoted_errors.rs:15:5
+   |
+LL |     0 - 1
+   |     ^^^^^ attempt to compute `0_u32 - 1_u32`, which would overflow
+   |
+note: the lint level is defined here
+  --> $DIR/promoted_errors.rs:11:20
+   |
+LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
+   |                    ^^^^^^^^^^^^^^^^^^^
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:21:5
+   |
+LL |     1 / 0
+   |     ^^^^^ attempt to divide `1_i32` by zero
+   |
+note: the lint level is defined here
+  --> $DIR/promoted_errors.rs:11:41
+   |
+LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
+   |                                         ^^^^^^^^^^^^^^^^^^^
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:27:5
+   |
+LL |     1 / (1 - 1)
+   |     ^^^^^^^^^^^ attempt to divide `1_i32` by zero
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:31:5
+   |
+LL |     1 / (false as i32)
+   |     ^^^^^^^^^^^^^^^^^^ attempt to divide `1_i32` by zero
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:35:5
+   |
+LL |     [1, 2, 3][4]
+   |     ^^^^^^^^^^^^ index out of bounds: the length is 3 but the index is 4
+
 warning: any use of this value will cause an error
-  --> $DIR/promoted_errors.rs:20:5
+  --> $DIR/promoted_errors.rs:21:5
    |
 LL |       1 / 0
    |       ^^^^^
    |       |
    |       attempt to divide `1_i32` by zero
-   |       inside `div_by_zero1` at $DIR/promoted_errors.rs:20:5
-   |       inside `X` at $DIR/promoted_errors.rs:41:29
+   |       inside `div_by_zero1` at $DIR/promoted_errors.rs:21:5
+   |       inside `X` at $DIR/promoted_errors.rs:46:29
 ...
 LL | / const X: () = {
 LL | |     let _x: &'static u32 = &overflow();
@@ -26,7 +68,7 @@ LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
 warning: any use of this value will cause an error
-  --> $DIR/promoted_errors.rs:41:28
+  --> $DIR/promoted_errors.rs:46:28
    |
 LL | / const X: () = {
 LL | |     let _x: &'static u32 = &overflow();
@@ -42,5 +84,5 @@ LL | | };
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-warning: 2 warnings emitted
+warning: 7 warnings emitted
 

src/test/ui/consts/const-eval/promoted_errors.opt_with_overflow_checks.stderr

@@ -1,3 +1,45 @@
+warning: this arithmetic operation will overflow
+  --> $DIR/promoted_errors.rs:15:5
+   |
+LL |     0 - 1
+   |     ^^^^^ attempt to compute `0_u32 - 1_u32`, which would overflow
+   |
+note: the lint level is defined here
+  --> $DIR/promoted_errors.rs:11:20
+   |
+LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
+   |                    ^^^^^^^^^^^^^^^^^^^
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:21:5
+   |
+LL |     1 / 0
+   |     ^^^^^ attempt to divide `1_i32` by zero
+   |
+note: the lint level is defined here
+  --> $DIR/promoted_errors.rs:11:41
+   |
+LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
+   |                                         ^^^^^^^^^^^^^^^^^^^
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:27:5
+   |
+LL |     1 / (1 - 1)
+   |     ^^^^^^^^^^^ attempt to divide `1_i32` by zero
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:31:5
+   |
+LL |     1 / (false as i32)
+   |     ^^^^^^^^^^^^^^^^^^ attempt to divide `1_i32` by zero
+
+warning: this operation will panic at runtime
+  --> $DIR/promoted_errors.rs:35:5
+   |
+LL |     [1, 2, 3][4]
+   |     ^^^^^^^^^^^^ index out of bounds: the length is 3 but the index is 4
+
 warning: any use of this value will cause an error
   --> $DIR/promoted_errors.rs:15:5
    |
@@ -6,7 +48,7 @@ LL |       0 - 1
    |       |
    |       attempt to compute `0_u32 - 1_u32`, which would overflow
    |       inside `overflow` at $DIR/promoted_errors.rs:15:5
-   |       inside `X` at $DIR/promoted_errors.rs:38:29
+   |       inside `X` at $DIR/promoted_errors.rs:43:29
 ...
 LL | / const X: () = {
 LL | |     let _x: &'static u32 = &overflow();
@@ -26,7 +68,7 @@ LL | #![warn(const_err, arithmetic_overflow, unconditional_panic)]
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
 warning: any use of this value will cause an error
-  --> $DIR/promoted_errors.rs:38:28
+  --> $DIR/promoted_errors.rs:43:28
    |
 LL | / const X: () = {
 LL | |     let _x: &'static u32 = &overflow();
@@ -41,5 +83,5 @@ LL | | };
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-warning: 2 warnings emitted
+warning: 7 warnings emitted
 

src/test/ui/consts/const-eval/promoted_errors.rs

@@ -15,20 +15,25 @@ const fn overflow() -> u32 {
     0 - 1
     //[opt_with_overflow_checks,noopt]~^ WARN any use of this value will cause an error
     //[opt_with_overflow_checks,noopt]~| WARN this was previously accepted by the compiler
+    //~^^^ WARN this arithmetic operation will overflow
 }
 const fn div_by_zero1() -> i32 {
     1 / 0
     //[opt]~^ WARN any use of this value will cause an error
     //[opt]~| WARN this was previously accepted by the compiler but is being phased out
+    //~^^^ WARN this operation will panic at runtime
 }
 const fn div_by_zero2() -> i32 {
     1 / (1 - 1)
+    //~^ WARN this operation will panic at runtime
 }
 const fn div_by_zero3() -> i32 {
     1 / (false as i32)
+    //~^ WARN this operation will panic at runtime
 }
 const fn oob() -> i32 {
     [1, 2, 3][4]
+    //~^ WARN this operation will panic at runtime
 }
 
 // An unused constant containing failing promoteds.

src/test/ui/consts/const-eval/union-const-eval-field.rs

@@ -28,6 +28,7 @@ const fn read_field3() -> Field3 {
     const FIELD3: Field3 = unsafe { UNION.field3 };
     //~^ ERROR it is undefined behavior to use this value
     FIELD3
+    //~^ ERROR erroneous constant used [E0080]
 }
 
 fn main() {

src/test/ui/consts/const-eval/union-const-eval-field.stderr

@@ -9,6 +9,12 @@ LL |     const FIELD3: Field3 = unsafe { UNION.field3 };
                __ __ __ __ __ __ __ __                         │ ░░░░░░░░
            }
 
-error: aborting due to previous error
+error[E0080]: erroneous constant used
+  --> $DIR/union-const-eval-field.rs:30:5
+   |
+LL |     FIELD3
+   |     ^^^^^^ referenced constant has errors
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/issue-56164.rs

@@ -1,5 +1,7 @@
 const fn foo() { (||{})() }
 //~^ ERROR cannot call non-const closure
+//~| ERROR erroneous constant used [const_err]
+//~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 const fn bad(input: fn()) {
     input()

src/test/ui/consts/issue-56164.stderr

@@ -8,11 +8,21 @@ LL | const fn foo() { (||{})() }
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error: function pointers are not allowed in const fn
-  --> $DIR/issue-56164.rs:5:5
+  --> $DIR/issue-56164.rs:7:5
    |
 LL |     input()
    |     ^^^^^^^
 
-error: aborting due to 2 previous errors
+error: erroneous constant used
+  --> $DIR/issue-56164.rs:1:18
+   |
+LL | const fn foo() { (||{})() }
+   |                  ^^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0015`.

src/test/ui/consts/issue-66693.rs

@@ -8,7 +8,10 @@
 //~^ ERROR: argument to `panic!()` in a const context must have type `&str`
 
 const fn _foo() {
-    panic!(&1); //~ ERROR: argument to `panic!()` in a const context must have type `&str`
+    panic!(&1);
+    //~^ ERROR: argument to `panic!()` in a const context must have type `&str`
+    //~| ERROR: erroneous constant used [const_err]
+    //~| WARNING: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 }
 
 // ensure that conforming panics don't cause an error

src/test/ui/consts/issue-66693.stderr

@@ -22,5 +22,15 @@ LL |     panic!(&1);
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 3 previous errors
+error: erroneous constant used
+  --> $DIR/issue-66693.rs:11:12
+   |
+LL |     panic!(&1);
+   |            ^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to 4 previous errors
 

src/test/ui/consts/promotion.rs

@@ -4,11 +4,13 @@
 //[opt_with_overflow_checks]compile-flags: -C overflow-checks=on -O
 
 // build-pass
-#[allow(arithmetic_overflow)]
 
 const fn assert_static<T>(_: &'static T) {}
 
-const fn fail() -> i32 { 1/0 }
+#[allow(unconditional_panic)]
+const fn fail() -> i32 {
+    1/0
+}
 const C: i32 = {
     // Promoted that fails to evaluate in dead code -- this must work
     // (for backwards compatibility reasons).

src/test/ui/recursion/issue-26548-recursion-via-normalize.rs

@@ -2,7 +2,6 @@
 //~| NOTE ...which requires computing layout of `core::option::Option<<S as Mirror>::It>`...
 //~| NOTE ...which requires computing layout of `core::option::Option<S>`...
 //~| NOTE ...which again requires computing layout of `S`, completing the cycle
-//~| NOTE cycle used when computing layout of `core::option::Option<S>`
 
 // build-fail
 
@@ -15,5 +14,6 @@ impl<T: ?Sized> Mirror for T {
 struct S(Option<<S as Mirror>::It>);
 
 fn main() {
+    //~^ NOTE cycle used when elaborating drops for `main`
     let _s = S(None);
 }

src/test/ui/recursion/issue-26548-recursion-via-normalize.stderr

@@ -3,7 +3,11 @@ error[E0391]: cycle detected when computing layout of `S`
    = note: ...which requires computing layout of `core::option::Option<<S as Mirror>::It>`...
    = note: ...which requires computing layout of `core::option::Option<S>`...
    = note: ...which again requires computing layout of `S`, completing the cycle
-   = note: cycle used when computing layout of `core::option::Option<S>`
+note: cycle used when elaborating drops for `main`
+  --> $DIR/issue-26548-recursion-via-normalize.rs:16:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
 
 error: aborting due to previous error
 

src/tools/clippy/tests/ui/crashes/ice-5497.stderr

@@ -0,0 +1,10 @@
+error: this operation will panic at runtime
+  --> $DIR/ice-5497.rs:9:22
+   |
+LL |     const OOB: i32 = [1][1] + T::OOB;
+   |                      ^^^^^^ index out of bounds: the length is 1 but the index is 1
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: aborting due to previous error
+