interpret: sanity-check that required_consts captures all consts that can fail

Author: RalfJung

Diff for: compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -375,6 +375,20 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error
 
+    #[inline]
+    fn all_required_consts_are_checked(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // Generally we expect required_consts to be properly filled, except for promoteds where
+        // storing these consts shows up negatively in benchmarks. A promoted can only be relevant
+        // if its parent MIR is relevant, and the consts in the promoted will be in the parent's
+        // `required_consts`, so we are still sure to catch any const-eval bugs, just a bit less
+        // directly.
+        if ecx.frame_idx() == 0 && ecx.frame().body.source.promoted.is_some() {
+            false
+        } else {
+            true
+        }
+    }
+
     #[inline(always)]
     fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         matches!(ecx.machine.check_alignment, CheckAlignment::Error)

Diff for: compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -792,15 +792,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         self.stack_mut().push(frame);
 
         // Make sure all the constants required by this frame evaluate successfully (post-monomorphization check).
-        if M::POST_MONO_CHECKS {
-            for &const_ in &body.required_consts {
-                let c = self
-                    .instantiate_from_current_frame_and_normalize_erasing_regions(const_.const_)?;
-                c.eval(*self.tcx, self.param_env, Some(const_.span)).map_err(|err| {
-                    err.emit_note(*self.tcx);
-                    err
-                })?;
-            }
+        for &const_ in &body.required_consts {
+            let c =
+                self.instantiate_from_current_frame_and_normalize_erasing_regions(const_.const_)?;
+            c.eval(*self.tcx, self.param_env, Some(const_.span)).map_err(|err| {
+                err.emit_note(*self.tcx);
+                err
+            })?;
         }
 
         // done
@@ -1149,8 +1147,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
         M::eval_mir_constant(self, *val, span, layout, |ecx, val, span, layout| {
             let const_val = val.eval(*ecx.tcx, ecx.param_env, span).map_err(|err| {
-                // FIXME: somehow this is reachable even when POST_MONO_CHECKS is on.
-                // Are we not always populating `required_consts`?
+                if M::all_required_consts_are_checked(self)
+                    && !matches!(err, ErrorHandled::TooGeneric(..))
+                {
+                    // Looks like the const is not captued by `required_consts`, that's bad.
+                    bug!("interpret const eval failure of {val:?} which is not in required_consts");
+                }
                 err.emit_note(*ecx.tcx);
                 err
             })?;

Diff for: compiler/rustc_const_eval/src/interpret/machine.rs

@@ -140,8 +140,9 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
     /// Should the machine panic on allocation failures?
     const PANIC_ON_ALLOC_FAIL: bool;
 
-    /// Should post-monomorphization checks be run when a stack frame is pushed?
-    const POST_MONO_CHECKS: bool = true;
+    /// Determines whether `eval_mir_constant` can never fail because all required consts have
+    /// already been checked before.
+    fn all_required_consts_are_checked(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
 
     /// Whether memory accesses should be alignment-checked.
     fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

Diff for: compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -900,6 +900,12 @@ impl<'mir, 'tcx: 'mir> rustc_const_eval::interpret::Machine<'mir, 'tcx> for Dumm
     type MemoryKind = !;
     const PANIC_ON_ALLOC_FAIL: bool = true;
 
+    #[inline(always)]
+    fn all_required_consts_are_checked(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // We want to just eval random consts in the program, so `eval_mir_const` can fail.
+        false
+    }
+
     #[inline(always)]
     fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         false // no reason to enforce alignment

Diff for: compiler/rustc_mir_transform/src/inline.rs

@@ -706,17 +706,7 @@ impl<'tcx> Inliner<'tcx> {
         });
 
         // Copy only unevaluated constants from the callee_body into the caller_body.
-        // Although we are only pushing `ConstKind::Unevaluated` consts to
-        // `required_consts`, here we may not only have `ConstKind::Unevaluated`
-        // because we are calling `instantiate_and_normalize_erasing_regions`.
-        caller_body.required_consts.extend(callee_body.required_consts.iter().copied().filter(
-            |&ct| match ct.const_ {
-                Const::Ty(_) => {
-                    bug!("should never encounter ty::UnevaluatedConst in `required_consts`")
-                }
-                Const::Val(..) | Const::Unevaluated(..) => true,
-            },
-        ));
+        caller_body.required_consts.extend(callee_body.required_consts);
     }
 
     fn make_call_args(

Diff for: compiler/rustc_mir_transform/src/required_consts.rs

@@ -1,6 +1,6 @@
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{Const, ConstOperand, Location};
-use rustc_middle::ty::ConstKind;
+use rustc_middle::ty;
 
 pub struct RequiredConstsVisitor<'a, 'tcx> {
     required_consts: &'a mut Vec<ConstOperand<'tcx>>,
@@ -14,14 +14,22 @@ impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
 
 impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
     fn visit_constant(&mut self, constant: &ConstOperand<'tcx>, _: Location) {
-        let const_ = constant.const_;
-        match const_ {
+        // Only unevaluated consts have to be added to `required_consts` as only those can possibly
+        // still have latent const-eval errors.
+        let is_required = match constant.const_ {
             Const::Ty(c) => match c.kind() {
-                ConstKind::Param(_) | ConstKind::Error(_) | ConstKind::Value(_) => {}
-                _ => bug!("only ConstKind::Param/Value should be encountered here, got {:#?}", c),
+                ty::ConstKind::Value(_) => false, // already a value, cannot error
+                ty::ConstKind::Param(_) | ty::ConstKind::Error(_) => true, // these are errors or could be replaced by errors
+                _ => bug!(
+                    "only ConstKind::Param/Value/Error should be encountered here, got {:#?}",
+                    c
+                ),
             },
-            Const::Unevaluated(..) => self.required_consts.push(*constant),
-            Const::Val(..) => {}
+            Const::Unevaluated(..) => true,
+            Const::Val(..) => false, // already a value, cannot error
+        };
+        if is_required {
+            self.required_consts.push(*constant);
         }
     }
 }

Diff for: src/tools/miri/src/machine.rs

@@ -872,6 +872,12 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
 
     const PANIC_ON_ALLOC_FAIL: bool = false;
 
+    #[inline(always)]
+    fn all_required_consts_are_checked(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // We want to catch any cases of missing required_consts
+        true
+    }
+
     #[inline(always)]
     fn enforce_alignment(ecx: &MiriInterpCx<'mir, 'tcx>) -> bool {
         ecx.machine.check_alignment != AlignmentCheck::None