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

compiler/rustc_const_eval/src/const_eval/dummy_machine.rs

@@ -46,6 +46,12 @@ impl<'mir, 'tcx: 'mir> interpret::Machine<'mir, 'tcx> for DummyMachine {
     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

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)

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -822,15 +822,13 @@ 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, 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, const_.span).map_err(|err| {
+                err.emit_note(*self.tcx);
+                err
+            })?;
         }
 
         // done
@@ -1181,8 +1179,12 @@ 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
             })?;

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;