Rollup merge of #151119 - reflect-pointers, r=oli-obk

Support pointers in type reflection

Tracking issue: rust-lang/rust#146922

This PR adds support for inspecting pointers `*const T` and `*mut T` through type reflection. It does so by adding the new `Pointer` struct + variant:

```rust
pub struct Pointer {
    /// The type of the value being pointed to.
    pub ty: TypeId,
    /// Whether this pointer is mutable or not.
    pub mutable: bool,
}
```

This can be gathered using `Type::of`, for example:

```rust
match const { Type::of::<*const u8>() }.kind {
    TypeKind::Pointer(pointer) => {
        assert_eq!(pointer.ty, TypeId::of::<u8>());
        assert!(!pointer.mutable);
    }
    _ => unreachable!(),
}
```

compiler/rustc_const_eval/src/const_eval/type_info.rs

@@ -1,7 +1,6 @@
 use rustc_abi::FieldIdx;
 use rustc_ast::Mutability;
 use rustc_hir::LangItem;
-use rustc_middle::mir::interpret::{CtfeProvenance, Scalar};
 use rustc_middle::span_bug;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_middle::ty::{self, Const, ScalarInt, Ty};
@@ -9,7 +8,8 @@ use rustc_span::{Symbol, sym};
 
 use crate::const_eval::CompileTimeMachine;
 use crate::interpret::{
-    Immediate, InterpCx, InterpResult, MPlaceTy, MemoryKind, Writeable, interp_ok,
+    CtfeProvenance, Immediate, InterpCx, InterpResult, MPlaceTy, MemoryKind, Scalar, Writeable,
+    interp_ok,
 };
 
 impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
@@ -112,11 +112,19 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
 
                             variant
                         }
+                        ty::RawPtr(ty, mutability) => {
+                            let (variant, variant_place) = downcast(sym::Pointer)?;
+                            let pointer_place =
+                                self.project_field(&variant_place, FieldIdx::ZERO)?;
+
+                            self.write_pointer_type_info(pointer_place, *ty, *mutability)?;
+
+                            variant
+                        }
                         ty::Adt(_, _)
                         | ty::Foreign(_)
                         | ty::Pat(_, _)
                         | ty::Slice(_)
-                        | ty::RawPtr(..)
                         | ty::FnDef(..)
                         | ty::FnPtr(..)
                         | ty::UnsafeBinder(..)
@@ -312,4 +320,30 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
         }
         interp_ok(())
     }
+
+    pub(crate) fn write_pointer_type_info(
+        &mut self,
+        place: impl Writeable<'tcx, CtfeProvenance>,
+        ty: Ty<'tcx>,
+        mutability: Mutability,
+    ) -> InterpResult<'tcx> {
+        // Iterate over all fields of `type_info::Pointer`.
+        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 pointer's inner type to the `ty` field.
+                sym::pointee => self.write_type_id(ty, &field_place)?,
+                // Write the boolean representing the pointer's mutability to the `mutable` field.
+                sym::mutable => {
+                    self.write_scalar(Scalar::from_bool(mutability.is_mut()), &field_place)?
+                }
+                other => span_bug!(self.tcx.def_span(field.did), "unimplemented field {other}"),
+            }
+        }
+
+        interp_ok(())
+    }
 }

library/core/src/mem/type_info.rs

@@ -57,6 +57,8 @@ pub enum TypeKind {
     Str(Str),
     /// References.
     Reference(Reference),
+    /// Pointers.
+    Pointer(Pointer),
     /// FIXME(#146922): add all the common types
     Other,
 }
@@ -146,3 +148,14 @@ pub struct Reference {
     /// Whether this reference is mutable or not.
     pub mutable: bool,
 }
+
+/// Compile-time type information about pointers.
+#[derive(Debug)]
+#[non_exhaustive]
+#[unstable(feature = "type_info", issue = "146922")]
+pub struct Pointer {
+    /// The type of the value being pointed to.
+    pub pointee: TypeId,
+    /// Whether this pointer is mutable or not.
+    pub mutable: bool,
+}

library/coretests/tests/mem/type_info.rs

@@ -107,7 +107,7 @@ fn test_references() {
         _ => unreachable!(),
     }
 
-    // Mutable pointer.
+    // Mutable references.
     match const { Type::of::<&mut u64>() }.kind {
         TypeKind::Reference(reference) => {
             assert_eq!(reference.pointee, TypeId::of::<u64>());
@@ -116,7 +116,7 @@ fn test_references() {
         _ => unreachable!(),
     }
 
-    // Wide pointer.
+    // Wide references.
     match const { Type::of::<&dyn Any>() }.kind {
         TypeKind::Reference(reference) => {
             assert_eq!(reference.pointee, TypeId::of::<dyn Any>());
@@ -125,3 +125,33 @@ fn test_references() {
         _ => unreachable!(),
     }
 }
+
+#[test]
+fn test_pointers() {
+    // Immutable pointer.
+    match const { Type::of::<*const u8>() }.kind {
+        TypeKind::Pointer(pointer) => {
+            assert_eq!(pointer.pointee, TypeId::of::<u8>());
+            assert!(!pointer.mutable);
+        }
+        _ => unreachable!(),
+    }
+
+    // Mutable pointer.
+    match const { Type::of::<*mut u64>() }.kind {
+        TypeKind::Pointer(pointer) => {
+            assert_eq!(pointer.pointee, TypeId::of::<u64>());
+            assert!(pointer.mutable);
+        }
+        _ => unreachable!(),
+    }
+
+    // Wide pointer.
+    match const { Type::of::<*const dyn Any>() }.kind {
+        TypeKind::Pointer(pointer) => {
+            assert_eq!(pointer.pointee, TypeId::of::<dyn Any>());
+            assert!(!pointer.mutable);
+        }
+        _ => unreachable!(),
+    }
+}