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

compiler/rustc_const_eval/src/interpret/discriminant.rs

@@ -44,7 +44,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         }
     }
 
-    /// Read discriminant, return the runtime value as well as the variant index.
+    /// Read discriminant, return the variant index.
     /// Can also legally be called on non-enums (e.g. through the discriminant_value intrinsic)!
     ///
     /// Will never return an uninhabited variant.
@@ -65,21 +65,17 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         // We use "tag" to refer to how the discriminant is encoded in memory, which can be either
         // straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
         let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout().variants {
+            Variants::Empty => {
+                throw_ub!(UninhabitedEnumVariantRead(None));
+            }
             Variants::Single { index } => {
-                // Do some extra checks on enums.
-                if ty.is_enum() {
-                    // Hilariously, `Single` is used even for 0-variant enums.
-                    // (See https://github.com/rust-lang/rust/issues/89765).
-                    if ty.ty_adt_def().unwrap().variants().is_empty() {
-                        throw_ub!(UninhabitedEnumVariantRead(index))
-                    }
+                if op.layout().is_uninhabited() {
                     // For consistency with `write_discriminant`, and to make sure that
                     // `project_downcast` cannot fail due to strange layouts, we declare immediate UB
-                    // for uninhabited variants.
-                    if op.layout().for_variant(self, index).is_uninhabited() {
-                        throw_ub!(UninhabitedEnumVariantRead(index))
-                    }
+                    // for uninhabited enums.
+                    throw_ub!(UninhabitedEnumVariantRead(Some(index)));
                 }
+                // Since the type is inhabited, there must be an index.
                 return interp_ok(index);
             }
             Variants::Multiple { tag, ref tag_encoding, tag_field, .. } => {
@@ -199,11 +195,13 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         // `uninhabited_enum_branching` MIR pass. It also ensures consistency with
         // `write_discriminant`.
         if op.layout().for_variant(self, index).is_uninhabited() {
-            throw_ub!(UninhabitedEnumVariantRead(index))
+            throw_ub!(UninhabitedEnumVariantRead(Some(index)))
         }
         interp_ok(index)
     }
 
+    /// Read discriminant, return the user-visible discriminant.
+    /// Can also legally be called on non-enums (e.g. through the discriminant_value intrinsic)!
     pub fn discriminant_for_variant(
         &self,
         ty: Ty<'tcx>,
@@ -243,6 +241,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         }
 
         match layout.variants {
+            abi::Variants::Empty => unreachable!("we already handled uninhabited types"),
             abi::Variants::Single { .. } => {
                 // The tag of a `Single` enum is like the tag of the niched
                 // variant: there's no tag as the discriminant is encoded

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -302,7 +302,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                     };
                 }
             }
-            Variants::Single { .. } => {}
+            Variants::Single { .. } | Variants::Empty => {}
         }
 
         // Now we know we are projecting to a field, so figure out which one.
@@ -344,6 +344,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                         // Inside a variant
                         PathElem::Field(def.variant(index).fields[FieldIdx::from_usize(field)].name)
                     }
+                    Variants::Empty => panic!("there is no field in Variants::Empty types"),
                     Variants::Multiple { .. } => bug!("we handled variants above"),
                 }
             }
@@ -1010,7 +1011,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
             }
             // Don't forget potential other variants.
             match &layout.variants {
-                Variants::Single { .. } => {
+                Variants::Single { .. } | Variants::Empty => {
                     // Fully handled above.
                 }
                 Variants::Multiple { variants, .. } => {

compiler/rustc_const_eval/src/interpret/visitor.rs

@@ -218,8 +218,8 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
                 // recurse with the inner type
                 self.visit_variant(v, idx, &inner)?;
             }
-            // For single-variant layouts, we already did anything there is to do.
-            Variants::Single { .. } => {}
+            // For single-variant layouts, we already did everything there is to do.
+            Variants::Single { .. } | Variants::Empty => {}
         }
 
         interp_ok(())

compiler/rustc_const_eval/src/util/check_validity_requirement.rs

@@ -155,6 +155,7 @@ fn check_validity_requirement_lax<'tcx>(
     }
 
     match &this.variants {
+        Variants::Empty => return Ok(false),
         Variants::Single { .. } => {
             // All fields of this single variant have already been checked above, there is nothing
             // else to do.