interpret/write_discriminant: when encoding niched variant, ensure the stored value matches

2 files changed, 14 insertions, 0 deletions

diff --git a/compiler/rustc_const_eval/src/errors.rs b/compiler/rustc_const_eval/src/errors.rs
index 4d2b1ba3eec..c8fdb06d291 100644
--- a/compiler/rustc_const_eval/src/errors.rs
+++ b/compiler/rustc_const_eval/src/errors.rs
@@ -509,6 +509,9 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             ScalarSizeMismatch(_) => const_eval_scalar_size_mismatch,
             UninhabitedEnumVariantWritten(_) => const_eval_uninhabited_enum_variant_written,
             UninhabitedEnumVariantRead(_) => const_eval_uninhabited_enum_variant_read,
+            InvalidNichedEnumVariantWritten { .. } => {
+                const_eval_invalid_niched_enum_variant_written
+            }
             AbiMismatchArgument { .. } => const_eval_incompatible_types,
             AbiMismatchReturn { .. } => const_eval_incompatible_return_types,
         }
@@ -597,6 +600,9 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
                 builder.arg("target_size", info.target_size);
                 builder.arg("data_size", info.data_size);
             }
+            InvalidNichedEnumVariantWritten { enum_ty } => {
+                builder.arg("ty", enum_ty.to_string());
+            }
             AbiMismatchArgument { caller_ty, callee_ty }
             | AbiMismatchReturn { caller_ty, callee_ty } => {
                 builder.arg("caller_ty", caller_ty.to_string());
diff --git a/compiler/rustc_const_eval/src/interpret/discriminant.rs b/compiler/rustc_const_eval/src/interpret/discriminant.rs
index bb8c17cf779..e951a77611c 100644
--- a/compiler/rustc_const_eval/src/interpret/discriminant.rs
+++ b/compiler/rustc_const_eval/src/interpret/discriminant.rs
@@ -85,6 +85,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     // Write result.
                     let niche_dest = self.project_field(dest, tag_field)?;
                     self.write_immediate(*tag_val, &niche_dest)?;
+                } else {
+                    // The untagged variant is implicitly encoded simply by having a value that is
+                    // outside the niche variants. But what if the data stored here does not
+                    // actually encode this variant? That would be bad! So let's double-check...
+                    let actual_variant = self.read_discriminant(&dest.to_op(self)?)?;
+                    if actual_variant != variant_index {
+                        throw_ub!(InvalidNichedEnumVariantWritten { enum_ty: dest.layout().ty });
+                    }
                 }
             }
         }