feat: support arrays in type reflection

compiler/rustc_const_eval/src/const_eval/type_info.rs

@@ -3,7 +3,7 @@ use rustc_hir::LangItem;
 use rustc_middle::mir::interpret::CtfeProvenance;
 use rustc_middle::span_bug;
 use rustc_middle::ty::layout::TyAndLayout;
-use rustc_middle::ty::{self, ScalarInt, Ty};
+use rustc_middle::ty::{self, Const, ScalarInt, Ty};
 use rustc_span::{Symbol, sym};
 
 use crate::const_eval::CompileTimeMachine;
@@ -56,14 +56,21 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
                             self.write_tuple_fields(tuple_place, fields, ty)?;
                             variant
                         }
+                        ty::Array(ty, len) => {
+                            let (variant, variant_place) = downcast(sym::Array)?;
+                            let array_place = self.project_field(&variant_place, FieldIdx::ZERO)?;
+
+                            self.write_array_type_info(array_place, *ty, *len)?;
+
+                            variant
+                        }
                         // For now just merge all primitives into one `Leaf` variant with no data
                         ty::Uint(_) | ty::Int(_) | ty::Float(_) | ty::Char | ty::Bool => {
                             downcast(sym::Leaf)?.0
                         }
                         ty::Adt(_, _)
                         | ty::Foreign(_)
                         | ty::Str
-                        | ty::Array(_, _)
                         | ty::Pat(_, _)
                         | ty::Slice(_)
                         | ty::RawPtr(..)
@@ -172,4 +179,28 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
         }
         interp_ok(())
     }
+
+    pub(crate) fn write_array_type_info(
+        &mut self,
+        place: impl Writeable<'tcx, CtfeProvenance>,
+        ty: Ty<'tcx>,
+        len: Const<'tcx>,
+    ) -> InterpResult<'tcx> {
+        // Iterate over all fields of `type_info::Array`.
+        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 array's elements to the `element_ty` field.
+                sym::element_ty => self.write_type_id(ty, &field_place)?,
+                // Write the length of the array to the `len` field.
+                sym::len => self.write_scalar(len.to_leaf(), &field_place)?,
+                other => span_bug!(self.tcx.def_span(field.did), "unimplemented field {other}"),
+            }
+        }
+
+        interp_ok(())
+    }
 }

compiler/rustc_span/src/symbol.rs

@@ -162,6 +162,7 @@ symbols! {
         Arc,
         ArcWeak,
         Argument,
+        Array,
         ArrayIntoIter,
         AsMut,
         AsRef,
@@ -939,6 +940,7 @@ symbols! {
         eii_impl,
         eii_internals,
         eii_shared_macro,
+        element_ty,
         emit,
         emit_enum,
         emit_enum_variant,

library/core/src/mem/type_info.rs

@@ -43,6 +43,8 @@ impl Type {
 pub enum TypeKind {
     /// Tuples.
     Tuple(Tuple),
+    /// Arrays.
+    Array(Array),
     /// Primitives
     /// FIXME(#146922): disambiguate further
     Leaf,
@@ -69,3 +71,14 @@ pub struct Field {
     /// Offset in bytes from the parent type
     pub offset: usize,
 }
+
+/// Compile-time type information about arrays.
+#[derive(Debug)]
+#[non_exhaustive]
+#[unstable(feature = "type_info", issue = "146922")]
+pub struct Array {
+    /// The type of each element in the array.
+    pub element_ty: TypeId,
+    /// The length of the array.
+    pub len: usize,
+}

library/coretests/tests/lib.rs

@@ -115,6 +115,7 @@
 #![feature(try_blocks)]
 #![feature(try_find)]
 #![feature(try_trait_v2)]
+#![feature(type_info)]
 #![feature(uint_bit_width)]
 #![feature(uint_gather_scatter_bits)]
 #![feature(unsize)]

library/coretests/tests/mem.rs

@@ -1,3 +1,5 @@
+mod type_info;
+
 use core::mem::*;
 use core::{array, ptr};
 use std::cell::Cell;

library/coretests/tests/mem/type_info.rs

@@ -0,0 +1,23 @@
+use std::any::TypeId;
+use std::mem::type_info::{Type, TypeKind};
+
+#[test]
+fn test_arrays() {
+    // Normal array.
+    match const { Type::of::<[u16; 4]>() }.kind {
+        TypeKind::Array(array) => {
+            assert_eq!(array.element_ty, TypeId::of::<u16>());
+            assert_eq!(array.len, 4);
+        }
+        _ => unreachable!(),
+    }
+
+    // Zero-length array.
+    match const { Type::of::<[bool; 0]>() }.kind {
+        TypeKind::Array(array) => {
+            assert_eq!(array.element_ty, TypeId::of::<bool>());
+            assert_eq!(array.len, 0);
+        }
+        _ => unreachable!(),
+    }
+}

tests/ui/reflection/dump.rs

@@ -22,6 +22,7 @@ struct Unsized {
 
 fn main() {
     println!("{:#?}", const { Type::of::<(u8, u8, ())>() }.kind);
+    println!("{:#?}", const { Type::of::<[u8; 2]>() }.kind);
     println!("{:#?}", const { Type::of::<Foo>() }.kind);
     println!("{:#?}", const { Type::of::<Bar>() }.kind);
     println!("{:#?}", const { Type::of::<&Unsized>() }.kind);

tests/ui/reflection/dump.run.stdout

@@ -16,6 +16,12 @@ Tuple(
         ],
     },
 )
+Array(
+    Array {
+        element_ty: TypeId(0x0596b48cc04376e64d5c788c2aa46bdb),
+        len: 2,
+    },
+)
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