add test for GVN issue; cleanup in dataflow_const_prop

2 files changed, 6 insertions, 7 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index 80d4bda4827..4653c9016c6 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -260,8 +260,12 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
         // This makes several assumptions about what layouts we will encounter; we match what
         // codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
         let inner_val: Immediate<_> = match (**self, self.layout.abi) {
-            // if the entire value is uninit, then so is the field (can happen in ConstProp)
+            // If the entire value is uninit, then so is the field (can happen in ConstProp).
             (Immediate::Uninit, _) => Immediate::Uninit,
+            // If the field is uninhabited, we can forget the data (can happen in ConstProp).
+            // `enum S { A(!), B, C }` is an example of an enum with Scalar layout that
+            // has an `Uninhabited` variant, which means this case is possible.
+            _ if layout.abi.is_uninhabited() => Immediate::Uninit,
             // the field contains no information, can be left uninit
             // (Scalar/ScalarPair can contain even aligned ZST, not just 1-ZST)
             _ if layout.is_zst() => Immediate::Uninit,
diff --git a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
index d5f22b2cdbc..ad12bce9b02 100644
--- a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
+++ b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
@@ -403,12 +403,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             operand,
             &mut |elem, op| match elem {
                 TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).ok(),
-                TrackElem::Variant(idx) => {
-                    if op.layout.for_variant(&self.ecx, idx).abi.is_uninhabited() {
-                        return None;
-                    }
-                    self.ecx.project_downcast(op, idx).ok()
-                }
+                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).ok(),
                 TrackElem::Discriminant => {
                     let variant = self.ecx.read_discriminant(op).ok()?;
                     let discr_value =