Rollup merge of #133702 - RalfJung:single-variant, r=oli-obk

Variants::Single: do not use invalid VariantIdx for uninhabited enums

~~Stacked on top of https://github.com/rust-lang/rust/pull/133681, only the last commit is new.~~

Currently, `Variants::Single` for an empty enum contains a `VariantIdx` of 0; looking that up in the enum variant list will ICE. That's quite confusing. So let's fix that by adding a new `Variants::Empty` case for types that have 0 variants.

try-job: i686-msvc

3 files changed, 5 insertions, 3 deletions

diff --git a/src/tools/miri/src/helpers.rs b/src/tools/miri/src/helpers.rs
index 1f7c60ad1bd..ef4543dcee8 100644
--- a/src/tools/miri/src/helpers.rs
+++ b/src/tools/miri/src/helpers.rs
@@ -605,7 +605,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                             // `UnsafeCell` action.
                             (self.unsafe_cell_action)(v)
                         }
-                        Variants::Single { .. } => {
+                        Variants::Single { .. } | Variants::Empty => {
                             // Proceed further, try to find where exactly that `UnsafeCell`
                             // is hiding.
                             self.walk_value(v)
diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/mir/eval.rs b/src/tools/rust-analyzer/crates/hir-ty/src/mir/eval.rs
index d7029651fc1..e3072d6ee79 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/mir/eval.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/mir/eval.rs
@@ -813,7 +813,7 @@ impl Evaluator<'_> {
                 ProjectionElem::Field(Either::Left(f)) => {
                     let layout = self.layout(&prev_ty)?;
                     let variant_layout = match &layout.variants {
-                        Variants::Single { .. } => &layout,
+                        Variants::Single { .. } | Variants::Empty => &layout,
                         Variants::Multiple { variants, .. } => {
                             &variants[match f.parent {
                                 hir_def::VariantId::EnumVariantId(it) => {
@@ -1638,6 +1638,7 @@ impl Evaluator<'_> {
             return Ok(0);
         };
         match &layout.variants {
+            Variants::Empty => unreachable!(),
             Variants::Single { index } => {
                 let r = self.const_eval_discriminant(self.db.enum_data(e).variants[index.0].0)?;
                 Ok(r)
@@ -1800,7 +1801,7 @@ impl Evaluator<'_> {
         }
         let layout = self.layout_adt(adt, subst)?;
         Ok(match &layout.variants {
-            Variants::Single { .. } => (layout.size.bytes_usize(), layout, None),
+            Variants::Single { .. } | Variants::Empty => (layout.size.bytes_usize(), layout, None),
             Variants::Multiple { variants, tag, tag_encoding, .. } => {
                 let enum_variant_id = match it {
                     VariantId::EnumVariantId(it) => it,
diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/utils.rs b/src/tools/rust-analyzer/crates/hir-ty/src/utils.rs
index 06719b09f73..42e7edaf0f4 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/utils.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/utils.rs
@@ -334,6 +334,7 @@ pub(crate) fn detect_variant_from_bytes<'a>(
     e: EnumId,
 ) -> Option<(EnumVariantId, &'a Layout)> {
     let (var_id, var_layout) = match &layout.variants {
+        hir_def::layout::Variants::Empty => unreachable!(),
         hir_def::layout::Variants::Single { index } => {
             (db.enum_data(e).variants[index.0].0, layout)
         }