require full validity when determining the discriminant of a value

Author: RalfJung

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -265,6 +265,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
             sym::discriminant_value => {
                 let place = self.deref_operand(&args[0])?;
+                if M::enforce_validity(self) {
+                    // This is 'using' the value, so make sure the validity invariant is satisfied.
+                    // (Also see https://github.com/rust-lang/rust/pull/89764.)
+                    self.validate_operand(&place.into())?;
+                }
+
                 let discr_val = self.read_discriminant(&place.into())?.0;
                 self.write_scalar(discr_val, dest)?;
             }

compiler/rustc_const_eval/src/interpret/step.rs

@@ -304,6 +304,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Discriminant(place) => {
                 let op = self.eval_place_to_op(place, None)?;
+                if M::enforce_validity(self) {
+                    // This is 'using' the value, so make sure the validity invariant is satisfied.
+                    // (Also see https://github.com/rust-lang/rust/pull/89764.)
+                    self.validate_operand(&op)?;
+                }
+
                 let discr_val = self.read_discriminant(&op)?.0;
                 self.write_scalar(discr_val, &dest)?;
             }