diff --git a/compiler/rustc_const_eval/messages.ftl b/compiler/rustc_const_eval/messages.ftl
index e7e8b2b36006d..9155bae263106 100644
--- a/compiler/rustc_const_eval/messages.ftl
+++ b/compiler/rustc_const_eval/messages.ftl
@@ -170,6 +170,10 @@ const_eval_invalid_meta =
invalid metadata in wide pointer: total size is bigger than largest supported object
const_eval_invalid_meta_slice =
invalid metadata in wide pointer: slice is bigger than largest supported object
+
+const_eval_invalid_niched_enum_variant_written =
+ trying to set discriminant of a {$ty} to the niched variant, but the value does not match
+
const_eval_invalid_str =
this string is not valid UTF-8: {$err}
const_eval_invalid_tag =
diff --git a/compiler/rustc_const_eval/src/errors.rs b/compiler/rustc_const_eval/src/errors.rs
index 4d2b1ba3eec89..c8fdb06d29158 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 bb8c17cf7791d..e951a77611c21 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 });
+ }
}
}
}
diff --git a/compiler/rustc_middle/src/mir/interpret/error.rs b/compiler/rustc_middle/src/mir/interpret/error.rs
index 0f69ab93452f7..4f915951163e3 100644
--- a/compiler/rustc_middle/src/mir/interpret/error.rs
+++ b/compiler/rustc_middle/src/mir/interpret/error.rs
@@ -356,6 +356,8 @@ pub enum UndefinedBehaviorInfo<'tcx> {
UninhabitedEnumVariantWritten(VariantIdx),
/// An uninhabited enum variant is projected.
UninhabitedEnumVariantRead(VariantIdx),
+ /// Trying to set discriminant to the niched variant, but the value does not match.
+ InvalidNichedEnumVariantWritten { enum_ty: Ty<'tcx> },
/// ABI-incompatible argument types.
AbiMismatchArgument { caller_ty: Ty<'tcx>, callee_ty: Ty<'tcx> },
/// ABI-incompatible return types.
diff --git a/src/tools/miri/tests/fail/enum-set-discriminant-niche-variant-wrong.rs b/src/tools/miri/tests/fail/enum-set-discriminant-niche-variant-wrong.rs
new file mode 100644
index 0000000000000..7097aa0c43ae3
--- /dev/null
+++ b/src/tools/miri/tests/fail/enum-set-discriminant-niche-variant-wrong.rs
@@ -0,0 +1,31 @@
+#![feature(core_intrinsics)]
+#![feature(custom_mir)]
+
+use std::intrinsics::mir::*;
+use std::num::NonZeroI32;
+
+// We define our own option type so that we can control the varian indices.
+#[allow(unused)]
+enum Option<T> {
+ None,
+ Some(T),
+}
+use Option::*;
+
+#[custom_mir(dialect = "runtime", phase = "optimized")]
+fn set_discriminant(ptr: &mut Option<NonZeroI32>) {
+ mir! {
+ {
+ // We set the discriminant to `Some`, which is a NOP since this is the niched variant.
+ // However, the enum is actually encoding `None` currently! That's not good...
+ SetDiscriminant(*ptr, 1);
+ //~^ ERROR: trying to set discriminant of a Option<std::num::NonZero<i32>> to the niched variant, but the value does not match
+ Return()
+ }
+ }
+}
+
+pub fn main() {
+ let mut v = None;
+ set_discriminant(&mut v);
+}
diff --git a/src/tools/miri/tests/fail/enum-set-discriminant-niche-variant-wrong.stderr b/src/tools/miri/tests/fail/enum-set-discriminant-niche-variant-wrong.stderr
new file mode 100644
index 0000000000000..a48a0a993daa1
--- /dev/null
+++ b/src/tools/miri/tests/fail/enum-set-discriminant-niche-variant-wrong.stderr
@@ -0,0 +1,20 @@
+error: Undefined Behavior: trying to set discriminant of a Option<std::num::NonZero<i32>> to the niched variant, but the value does not match
+ --> $DIR/enum-set-discriminant-niche-variant-wrong.rs:LL:CC
+ |
+LL | SetDiscriminant(*ptr, 1);
+ | ^^^^^^^^^^^^^^^^^^^^^^^^ trying to set discriminant of a Option<std::num::NonZero<i32>> to the niched variant, but the value does not match
+ |
+ = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+ = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+ = note: BACKTRACE:
+ = note: inside `set_discriminant` at $DIR/enum-set-discriminant-niche-variant-wrong.rs:LL:CC
+note: inside `main`
+ --> $DIR/enum-set-discriminant-niche-variant-wrong.rs:LL:CC
+ |
+LL | set_discriminant(&mut v);
+ | ^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+