Auto merge of rust-lang#133702 - RalfJung:single-variant, r=<try>

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

~~Stacked on top of rust-lang#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

Author: bors

compiler/rustc_abi/src/callconv.rs

@@ -206,7 +206,7 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
                 let (mut result, mut total) = from_fields_at(*self, Size::ZERO)?;
 
                 match &self.variants {
-                    abi::Variants::Single { .. } => {}
+                    abi::Variants::Single { .. } | abi::Variants::Empty => {}
                     abi::Variants::Multiple { variants, .. } => {
                         // Treat enum variants like union members.
                         // HACK(eddyb) pretend the `enum` field (discriminant)

compiler/rustc_abi/src/layout.rs

@@ -213,8 +213,9 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         &self,
     ) -> LayoutData<FieldIdx, VariantIdx> {
         let dl = self.cx.data_layout();
+        // This is also used for uninhabited enums, so we use `Variants::Empty`.
         LayoutData {
-            variants: Variants::Single { index: VariantIdx::new(0) },
+            variants: Variants::Empty,
             fields: FieldsShape::Primitive,
             backend_repr: BackendRepr::Uninhabited,
             largest_niche: None,
@@ -1004,8 +1005,8 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             Variants::Multiple { tag, tag_encoding, tag_field, .. } => {
                 Variants::Multiple { tag, tag_encoding, tag_field, variants: best_layout.variants }
             }
-            Variants::Single { .. } => {
-                panic!("encountered a single-variant enum during multi-variant layout")
+            Variants::Single { .. } | Variants::Empty => {
+                panic!("encountered a single-variant or empty enum during multi-variant layout")
             }
         };
         Ok(best_layout.layout)

compiler/rustc_abi/src/lib.rs

@@ -1504,10 +1504,12 @@ impl BackendRepr {
 #[derive(PartialEq, Eq, Hash, Clone, Debug)]
 #[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
 pub enum Variants<FieldIdx: Idx, VariantIdx: Idx> {
+    /// A type with no valid variants. Must be uninhabited.
+    Empty,
+
     /// Single enum variants, structs/tuples, unions, and all non-ADTs.
     Single {
-        /// Always 0 for non-enums/generators.
-        /// For enums without a variant, this is an invalid index!
+        /// Always `0` for types that cannot have multiple variants.
         index: VariantIdx,
     },
 

compiler/rustc_codegen_cranelift/src/discriminant.rs

@@ -18,6 +18,7 @@ pub(crate) fn codegen_set_discriminant<'tcx>(
         return;
     }
     match layout.variants {
+        Variants::Empty => unreachable!("we already handled uninhabited types"),
         Variants::Single { index } => {
             assert_eq!(index, variant_index);
         }
@@ -85,6 +86,7 @@ pub(crate) fn codegen_get_discriminant<'tcx>(
     }
 
     let (tag_scalar, tag_field, tag_encoding) = match &layout.variants {
+        Variants::Empty => unreachable!("we already handled uninhabited types"),
         Variants::Single { index } => {
             let discr_val = layout
                 .ty

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/cpp_like.rs

@@ -212,21 +212,17 @@ pub(super) fn build_enum_type_di_node<'ll, 'tcx>(
         ),
         |cx, enum_type_di_node| {
             match enum_type_and_layout.variants {
-                Variants::Single { index: variant_index } => {
-                    if enum_adt_def.variants().is_empty() {
-                        // Uninhabited enums have Variants::Single. We don't generate
-                        // any members for them.
-                        return smallvec![];
-                    }
-
-                    build_single_variant_union_fields(
-                        cx,
-                        enum_adt_def,
-                        enum_type_and_layout,
-                        enum_type_di_node,
-                        variant_index,
-                    )
+                Variants::Empty => {
+                    // We don't generate any members for uninhabited types.
+                    return smallvec![];
                 }
+                Variants::Single { index: variant_index } => build_single_variant_union_fields(
+                    cx,
+                    enum_adt_def,
+                    enum_type_and_layout,
+                    enum_type_di_node,
+                    variant_index,
+                ),
                 Variants::Multiple {
                     tag_encoding: TagEncoding::Direct,
                     ref variants,
@@ -303,6 +299,7 @@ pub(super) fn build_coroutine_di_node<'ll, 'tcx>(
                 )
             }
             Variants::Single { .. }
+            | Variants::Empty
             | Variants::Multiple { tag_encoding: TagEncoding::Niche { .. }, .. } => {
                 bug!(
                     "Encountered coroutine with non-direct-tag layout: {:?}",

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs

@@ -392,7 +392,7 @@ fn compute_discriminant_value<'ll, 'tcx>(
     variant_index: VariantIdx,
 ) -> DiscrResult {
     match enum_type_and_layout.layout.variants() {
-        &Variants::Single { .. } => DiscrResult::NoDiscriminant,
+        &Variants::Single { .. } | &Variants::Empty => DiscrResult::NoDiscriminant,
         &Variants::Multiple { tag_encoding: TagEncoding::Direct, .. } => DiscrResult::Value(
             enum_type_and_layout.ty.discriminant_for_variant(cx.tcx, variant_index).unwrap().val,
         ),

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs

@@ -358,8 +358,8 @@ fn build_discr_member_di_node<'ll, 'tcx>(
     let containing_scope = enum_or_coroutine_type_di_node;
 
     match enum_or_coroutine_type_and_layout.layout.variants() {
-        // A single-variant enum has no discriminant.
-        &Variants::Single { .. } => None,
+        // A single-variant or no-variant enum has no discriminant.
+        &Variants::Single { .. } | &Variants::Empty => None,
 
         &Variants::Multiple { tag_field, .. } => {
             let tag_base_type = tag_base_type(cx.tcx, enum_or_coroutine_type_and_layout);

compiler/rustc_codegen_llvm/src/type_of.rs

@@ -38,7 +38,7 @@ fn uncached_llvm_type<'a, 'tcx>(
             if let (&ty::Adt(def, _), &Variants::Single { index }) =
                 (layout.ty.kind(), &layout.variants)
             {
-                if def.is_enum() && !def.variants().is_empty() {
+                if def.is_enum() {
                     write!(&mut name, "::{}", def.variant(index).name).unwrap();
                 }
             }

compiler/rustc_codegen_ssa/src/debuginfo/mod.rs

@@ -65,8 +65,8 @@ fn tag_base_type_opt<'tcx>(
     });
 
     match enum_type_and_layout.layout.variants() {
-        // A single-variant enum has no discriminant.
-        Variants::Single { .. } => None,
+        // A single-variant or no-variant enum has no discriminant.
+        Variants::Single { .. } | Variants::Empty => None,
 
         Variants::Multiple { tag_encoding: TagEncoding::Niche { .. }, tag, .. } => {
             // Niche tags are always normalized to unsized integers of the correct size.

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -243,6 +243,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
             return bx.cx().const_poison(cast_to);
         }
         let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants {
+            Variants::Empty => unreachable!("we already handled uninhabited types"),
             Variants::Single { index } => {
                 let discr_val = self
                     .layout
@@ -365,9 +366,9 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
             return;
         }
         match self.layout.variants {
-            Variants::Single { index } => {
-                assert_eq!(index, variant_index);
-            }
+            Variants::Empty => unreachable!("we already handled uninhabited types"),
+            Variants::Single { index } => assert_eq!(index, variant_index),
+
             Variants::Multiple { tag_encoding: TagEncoding::Direct, tag_field, .. } => {
                 let ptr = self.project_field(bx, tag_field);
                 let to =