Rollup merge of #151118 - BD103:reflect-slices, r=oli-obk

Support slices in type reflection

Tracking issue: rust-lang/rust#146922

This PR adds support for inspecting slices `[T]` through type reflection. It does so by adding the new `Slice` struct + variant, which is very similar to `Array` but without the `len` field:

```rust
pub struct Slice {
    pub element_ty: TypeId,
}
```

Here is an example reflecting a slice:

```rust
match const { Type::of::<[u8]>() }.kind {
    TypeKind::Slice(slice) => assert_eq!(slice.element_ty, TypeId::of::<u8>()),
    _ => unreachable!(),
}
```

In addition to this, I also re-arranged the type info unit tests.

r? @oli-obk

compiler/rustc_const_eval/src/const_eval/type_info.rs

@@ -66,6 +66,14 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
 
                             variant
                         }
+                        ty::Slice(ty) => {
+                            let (variant, variant_place) = downcast(sym::Slice)?;
+                            let slice_place = self.project_field(&variant_place, FieldIdx::ZERO)?;
+
+                            self.write_slice_type_info(slice_place, *ty)?;
+
+                            variant
+                        }
                         ty::Bool => {
                             let (variant, _variant_place) = downcast(sym::Bool)?;
                             variant
@@ -124,7 +132,6 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
                         ty::Adt(_, _)
                         | ty::Foreign(_)
                         | ty::Pat(_, _)
-                        | ty::Slice(_)
                         | ty::FnDef(..)
                         | ty::FnPtr(..)
                         | ty::UnsafeBinder(..)
@@ -254,6 +261,27 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
         interp_ok(())
     }
 
+    pub(crate) fn write_slice_type_info(
+        &mut self,
+        place: impl Writeable<'tcx, CtfeProvenance>,
+        ty: Ty<'tcx>,
+    ) -> InterpResult<'tcx> {
+        // Iterate over all fields of `type_info::Slice`.
+        for (field_idx, field) in
+            place.layout().ty.ty_adt_def().unwrap().non_enum_variant().fields.iter_enumerated()
+        {
+            let field_place = self.project_field(&place, field_idx)?;
+
+            match field.name {
+                // Write the `TypeId` of the slice's elements to the `element_ty` field.
+                sym::element_ty => self.write_type_id(ty, &field_place)?,
+                other => span_bug!(self.tcx.def_span(field.did), "unimplemented field {other}"),
+            }
+        }
+
+        interp_ok(())
+    }
+
     fn write_int_type_info(
         &mut self,
         place: impl Writeable<'tcx, CtfeProvenance>,

compiler/rustc_span/src/symbol.rs

@@ -362,6 +362,7 @@ symbols! {
         Send,
         SeqCst,
         Sized,
+        Slice,
         SliceIndex,
         SliceIter,
         Some,

library/core/src/mem/type_info.rs

@@ -45,6 +45,8 @@ pub enum TypeKind {
     Tuple(Tuple),
     /// Arrays.
     Array(Array),
+    /// Slices.
+    Slice(Slice),
     /// Primitive boolean type.
     Bool(Bool),
     /// Primitive character type.
@@ -94,6 +96,15 @@ pub struct Array {
     pub len: usize,
 }
 
+/// Compile-time type information about slices.
+#[derive(Debug)]
+#[non_exhaustive]
+#[unstable(feature = "type_info", issue = "146922")]
+pub struct Slice {
+    /// The type of each element in the slice.
+    pub element_ty: TypeId,
+}
+
 /// Compile-time type information about `bool`.
 #[derive(Debug)]
 #[non_exhaustive]

library/coretests/tests/mem/type_info.rs

@@ -22,6 +22,14 @@ fn test_arrays() {
     }
 }
 
+#[test]
+fn test_slices() {
+    match const { Type::of::<[usize]>() }.kind {
+        TypeKind::Slice(slice) => assert_eq!(slice.element_ty, TypeId::of::<usize>()),
+        _ => unreachable!(),
+    }
+}
+
 #[test]
 fn test_tuples() {
     fn assert_tuple_arity<T: 'static, const N: usize>() {

tests/ui/reflection/dump.bit32.run.stdout

@@ -194,7 +194,11 @@ Type {
     size: None,
 }
 Type {
-    kind: Other,
+    kind: Slice(
+        Slice {
+            element_ty: TypeId(0x0596b48cc04376e64d5c788c2aa46bdb),
+        },
+    ),
     size: None,
 }
 Type {

tests/ui/reflection/dump.bit64.run.stdout

@@ -194,7 +194,11 @@ Type {
     size: None,
 }
 Type {
-    kind: Other,
+    kind: Slice(
+        Slice {
+            element_ty: TypeId(0x0596b48cc04376e64d5c788c2aa46bdb),
+        },
+    ),
     size: None,
 }
 Type {