Implement const-eval support for function pointer types and add tests

- Implement handling of FnPtr TypeKind in const-eval, including:
  - Unsafety flag (safe vs unsafe fn)
  - ABI variants (Rust, Named(C), Named(custom))
  - Input and output types
  - Variadic function pointers
- Add const-eval tests covering:
  - Basic Rust fn() pointers
  - Unsafe fn() pointers
  - Extern C and custom ABI pointers
  - Functions with multiple inputs and output types
  - Variadic functions
- Use const TypeId checks to verify correctness of inputs, outputs, and payloads

Author: jasper3108

compiler/rustc_const_eval/src/const_eval/type_info.rs

@@ -2,12 +2,12 @@ mod adt;
 
 use std::borrow::Cow;
 
-use rustc_abi::{FieldIdx, VariantIdx};
+use rustc_abi::{ExternAbi, FieldIdx, VariantIdx};
 use rustc_ast::Mutability;
 use rustc_hir::LangItem;
 use rustc_middle::span_bug;
 use rustc_middle::ty::layout::TyAndLayout;
-use rustc_middle::ty::{self, Const, ScalarInt, Ty};
+use rustc_middle::ty::{self, Const, FnHeader, FnSigTys, ScalarInt, Ty, TyCtxt};
 use rustc_span::{Symbol, sym};
 
 use crate::const_eval::CompileTimeMachine;
@@ -188,10 +188,21 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
                             self.write_dyn_trait_type_info(dyn_place, *predicates, *region)?;
                             variant
                         }
+                        ty::FnPtr(sig, fn_header) => {
+                            let (variant, variant_place) =
+                                self.downcast(&field_dest, sym::FnPtr)?;
+                            let fn_ptr_place =
+                                self.project_field(&variant_place, FieldIdx::ZERO)?;
+
+                            // FIXME: handle lifetime bounds
+                            let sig = sig.skip_binder();
+
+                            self.write_fn_ptr_type_info(fn_ptr_place, &sig, fn_header)?;
+                            variant
+                        }
                         ty::Foreign(_)
                         | ty::Pat(_, _)
                         | ty::FnDef(..)
-                        | ty::FnPtr(..)
                         | ty::UnsafeBinder(..)
                         | ty::Closure(..)
                         | ty::CoroutineClosure(..)
@@ -402,6 +413,65 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
         interp_ok(())
     }
 
+    pub(crate) fn write_fn_ptr_type_info(
+        &mut self,
+        place: impl Writeable<'tcx, CtfeProvenance>,
+        sig: &FnSigTys<TyCtxt<'tcx>>,
+        fn_header: &FnHeader<TyCtxt<'tcx>>,
+    ) -> InterpResult<'tcx> {
+        let FnHeader { safety, c_variadic, abi, .. } = fn_header;
+
+        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 {
+                sym::unsafety => {
+                    self.write_scalar(Scalar::from_bool(safety.is_unsafe()), &field_place)?;
+                }
+                sym::abi => match abi {
+                    ExternAbi::C { .. } => {
+                        let (rust_variant, _rust_place) =
+                            self.downcast(&field_place, sym::ExternC)?;
+                        self.write_discriminant(rust_variant, &field_place)?;
+                    }
+                    ExternAbi::Rust => {
+                        let (rust_variant, _rust_place) =
+                            self.downcast(&field_place, sym::ExternRust)?;
+                        self.write_discriminant(rust_variant, &field_place)?;
+                    }
+                    other_abi => {
+                        let (variant, variant_place) = self.downcast(&field_place, sym::Named)?;
+                        let str_place = self.allocate_str_dedup(other_abi.as_str())?;
+                        let str_ref = self.mplace_to_ref(&str_place)?;
+                        let payload = self.project_field(&variant_place, FieldIdx::ZERO)?;
+                        self.write_immediate(*str_ref, &payload)?;
+                        self.write_discriminant(variant, &field_place)?;
+                    }
+                },
+                sym::inputs => {
+                    let inputs = sig.inputs();
+                    self.allocate_fill_and_write_slice_ptr(
+                        field_place,
+                        inputs.len() as _,
+                        |this, i, place| this.write_type_id(inputs[i as usize], &place),
+                    )?;
+                }
+                sym::output => {
+                    let output = sig.output();
+                    self.write_type_id(output, &field_place)?;
+                }
+                sym::variadic => {
+                    self.write_scalar(Scalar::from_bool(*c_variadic), &field_place)?;
+                }
+                other => span_bug!(self.tcx.def_span(field.did), "unimplemented field {other}"),
+            }
+        }
+
+        interp_ok(())
+    }
+
     pub(crate) fn write_pointer_type_info(
         &mut self,
         place: impl Writeable<'tcx, CtfeProvenance>,

compiler/rustc_span/src/symbol.rs

@@ -242,6 +242,8 @@ symbols! {
         Equal,
         Err,
         Error,
+        ExternC,
+        ExternRust,
         File,
         FileType,
         Float,
@@ -250,6 +252,7 @@ symbols! {
         Fn,
         FnMut,
         FnOnce,
+        FnPtr,
         Formatter,
         Forward,
         From,
@@ -303,6 +306,7 @@ symbols! {
         Mutex,
         MutexGuard,
         N,
+        Named,
         NonNull,
         NonZero,
         None,
@@ -1290,6 +1294,7 @@ symbols! {
         inline_const,
         inline_const_pat,
         inout,
+        inputs,
         instant_now,
         instruction_set,
         integer_: "integer", // underscore to avoid clashing with the function `sym::integer` below
@@ -1660,6 +1665,7 @@ symbols! {
         os_string_as_os_str,
         other,
         out,
+        output,
         overflow_checks,
         overlapping_marker_traits,
         owned_box,
@@ -2440,6 +2446,7 @@ symbols! {
         unsafe_no_drop_flag,
         unsafe_pinned,
         unsafe_unpin,
+        unsafety,
         unsize,
         unsized_const_param_ty,
         unsized_const_params,
@@ -2484,6 +2491,7 @@ symbols! {
         value,
         values,
         var,
+        variadic,
         variant_count,
         variants,
         vec,

library/core/src/mem/type_info.rs

@@ -75,6 +75,8 @@ pub enum TypeKind {
     Reference(Reference),
     /// Pointers.
     Pointer(Pointer),
+    /// Function pointers.
+    FnPtr(FnPtr),
     /// FIXME(#146922): add all the common types
     Other,
 }
@@ -305,3 +307,39 @@ pub struct Pointer {
     /// Whether this pointer is mutable or not.
     pub mutable: bool,
 }
+
+#[derive(Debug)]
+#[unstable(feature = "type_info", issue = "146922")]
+/// Function pointer, e.g. fn(u8),
+pub struct FnPtr {
+    /// Unsafety, true is unsafe
+    pub unsafety: bool,
+
+    /// Abi, e.g. extern "C"
+    pub abi: Abi,
+
+    /// Function inputs
+    pub inputs: &'static [TypeId],
+
+    /// Function return type, default is TypeId::of::<()>
+    pub output: TypeId,
+
+    /// Vardiadic function, e.g. extern "C" fn add(n: usize, mut args: ...);
+    pub variadic: bool,
+}
+
+#[derive(Debug, Default)]
+#[non_exhaustive]
+#[unstable(feature = "type_info", issue = "146922")]
+/// Abi of [FnPtr]
+pub enum Abi {
+    /// Named abi, e.g. extern "custom", "stdcall" etc.
+    Named(&'static str),
+
+    /// Default
+    #[default]
+    ExternRust,
+
+    /// C-calling convention
+    ExternC,
+}

library/coretests/tests/mem.rs

@@ -1,3 +1,4 @@
+mod fn_ptr;
 mod type_info;
 
 use core::mem::*;

library/coretests/tests/mem/fn_ptr.rs

@@ -0,0 +1,169 @@
+use std::any::TypeId;
+use std::mem::type_info::{Abi, FnPtr, Type, TypeKind};
+
+const STRING_TY: TypeId = const { TypeId::of::<String>() };
+const U8_TY: TypeId = const { TypeId::of::<u8>() };
+const _U8_REF_TY: TypeId = const { TypeId::of::<&u8>() };
+const UNIT_TY: TypeId = const { TypeId::of::<()>() };
+
+#[test]
+fn test_fn_ptrs() {
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternRust,
+        inputs: &[],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<fn()>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+}
+#[test]
+fn test_ref() {
+    const {
+        // references are tricky because the lifetimes give the references different type ids
+        // so we check the pointees instead
+        let TypeKind::FnPtr(FnPtr {
+            unsafety: false,
+            abi: Abi::ExternRust,
+            inputs: &[ty1, ty2],
+            output,
+            variadic: false,
+        }) = (const { Type::of::<fn(&u8, &u8)>().kind })
+        else {
+            panic!();
+        };
+        if output != UNIT_TY {
+            panic!();
+        }
+        let TypeKind::Reference(reference) = ty1.info().kind else {
+            panic!();
+        };
+        if reference.pointee != U8_TY {
+            panic!();
+        }
+        let TypeKind::Reference(reference) = ty2.info().kind else {
+            panic!();
+        };
+        if reference.pointee != U8_TY {
+            panic!();
+        }
+    }
+}
+
+#[test]
+fn test_unsafe() {
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: true,
+        abi: Abi::ExternRust,
+        inputs: &[],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<unsafe fn()>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+}
+#[test]
+fn test_abi() {
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternRust,
+        inputs: &[],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<extern "Rust" fn()>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternC,
+        inputs: &[],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<extern "C" fn()>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: true,
+        abi: Abi::Named("system"),
+        inputs: &[],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<unsafe extern "system" fn()>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+}
+
+#[test]
+fn test_inputs() {
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternRust,
+        inputs: &[ty1, ty2],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<fn(String, u8)>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+    assert_eq!(ty1, STRING_TY);
+    assert_eq!(ty2, U8_TY);
+
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternRust,
+        inputs: &[ty1, ty2],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<fn(val: String, p2: u8)>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, UNIT_TY);
+    assert_eq!(ty1, STRING_TY);
+    assert_eq!(ty2, U8_TY);
+}
+
+#[test]
+fn test_output() {
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternRust,
+        inputs: &[],
+        output,
+        variadic: false,
+    }) = (const { Type::of::<fn() -> u8>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, U8_TY);
+}
+
+#[test]
+fn test_variadic() {
+    let TypeKind::FnPtr(FnPtr {
+        unsafety: false,
+        abi: Abi::ExternC,
+        inputs: [ty1],
+        output,
+        variadic: true,
+    }) = &(const { Type::of::<extern "C" fn(u8, ...)>().kind })
+    else {
+        panic!();
+    };
+    assert_eq!(output, &UNIT_TY);
+    assert_eq!(*ty1, U8_TY);
+}