Rollup merge of #94271 - RalfJung:downcast, r=oli-obk

Miri: extend comments on downcast operation

r? `@oli-obk`

2 files changed, 6 insertions, 2 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index bc4dca4c146..cd147b03bca 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -405,10 +405,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         op: &OpTy<'tcx, M::PointerTag>,
         variant: VariantIdx,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
-        // Downcasts only change the layout
         Ok(match op.try_as_mplace() {
             Ok(ref mplace) => self.mplace_downcast(mplace, variant)?.into(),
             Err(..) => {
+                // Downcasts only change the layout.
+                // (In particular, no check about whether this is even the active variant -- that's by design,
+                // see https://github.com/rust-lang/rust/issues/93688#issuecomment-1032929496.)
                 let layout = op.layout.for_variant(self, variant);
                 OpTy { layout, ..*op }
             }
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index e9b2df53a33..b1784b12c65 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -479,7 +479,9 @@ where
         base: &MPlaceTy<'tcx, M::PointerTag>,
         variant: VariantIdx,
     ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
-        // Downcasts only change the layout
+        // Downcasts only change the layout.
+        // (In particular, no check about whether this is even the active variant -- that's by design,
+        // see https://github.com/rust-lang/rust/issues/93688#issuecomment-1032929496.)
         assert!(!base.meta.has_meta());
         Ok(MPlaceTy { layout: base.layout.for_variant(self, variant), ..*base })
     }