make no-variant types a dedicated Variants variant

compiler/rustc_abi/src/lib.rs

@@ -1504,11 +1504,13 @@ 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 `Some(0)` for types without variants (i.e., everything except for `!`, enums, and
-        /// generators). `None` indicates an uninhabited type; this is used for zero-variant enums.
-        index: Option<VariantIdx>,
+        /// Always `0` for types that cannot have multiple variants.
+        index: VariantIdx,
     },
 
     /// Enum-likes with more than one variant: each variant comes with
@@ -1706,7 +1708,7 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
         let size = scalar.size(cx);
         let align = scalar.align(cx);
         LayoutData {
-            variants: Variants::Single { index: Some(VariantIdx::new(0)) },
+            variants: Variants::Single { index: VariantIdx::new(0) },
             fields: FieldsShape::Primitive,
             backend_repr: BackendRepr::Scalar(scalar),
             largest_niche,