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

@@ -1,9 +1,9 @@
-use rustc_abi::FieldIdx;
+use rustc_abi::{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, ScalarInt, Ty};
 use rustc_span::{Symbol, sym};
 
 use crate::const_eval::CompileTimeMachine;
@@ -13,11 +13,59 @@ use crate::interpret::{
 };
 
 impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
+    // FIXME: Merge with #151142
+    fn downcast(
+        &self,
+        place: &(impl Writeable<'tcx, CtfeProvenance> + 'tcx),
+        name: Symbol,
+    ) -> InterpResult<'tcx, (VariantIdx, impl Writeable<'tcx, CtfeProvenance> + 'tcx)> {
+        let variants = place.layout().ty.ty_adt_def().unwrap().variants();
+        let variant_id = variants
+            .iter_enumerated()
+            .find(|(_idx, var)| var.name == name)
+            .unwrap_or_else(|| panic!("got {name} but expected one of {variants:#?}"))
+            .0;
+
+        interp_ok((variant_id, self.project_downcast(place, variant_id)?))
+    }
+
+    // FIXME: Merge with #151142
+    // A general method to write an array to a static slice place.
+    fn allocate_fill_and_write_slice_ptr(
+        &mut self,
+        slice_place: impl Writeable<'tcx, CtfeProvenance>,
+        len: u64,
+        writer: impl Fn(&mut Self, /* index */ u64, MPlaceTy<'tcx>) -> InterpResult<'tcx>,
+    ) -> InterpResult<'tcx> {
+        // Array element type
+        let field_ty = slice_place
+            .layout()
+            .ty
+            .builtin_deref(false)
+            .unwrap()
+            .sequence_element_type(self.tcx.tcx);
+
+        // Allocate an array
+        let array_layout = self.layout_of(Ty::new_array(self.tcx.tcx, field_ty, len))?;
+        let array_place = self.allocate(array_layout, MemoryKind::Stack)?;
+
+        // Fill the array fields
+        let mut field_places = self.project_array_fields(&array_place)?;
+        while let Some((i, place)) = field_places.next(self)? {
+            writer(self, i, place)?;
+        }
+
+        // Write the slice pointing to the array
+        let array_place = array_place.map_provenance(CtfeProvenance::as_immutable);
+        let ptr = Immediate::new_slice(array_place.ptr(), len, self);
+        self.write_immediate(ptr, &slice_place)
+    }
+
     /// Writes a `core::mem::type_info::TypeInfo` for a given type, `ty` to the given place.
     pub(crate) fn write_type_info(
         &mut self,
         ty: Ty<'tcx>,
-        dest: &impl Writeable<'tcx, CtfeProvenance>,
+        dest: &(impl Writeable<'tcx, CtfeProvenance> + 'tcx),
     ) -> InterpResult<'tcx> {
         let ty_struct = self.tcx.require_lang_item(LangItem::Type, self.tcx.span);
         let ty_struct = self.tcx.type_of(ty_struct).no_bound_vars().unwrap();
@@ -135,11 +183,24 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
                             self.write_dyn_trait_type_info(dyn_place, *predicates, *region)?;
                             variant
                         }
+                        ty::FnPtr(binder, FnHeader { safety, c_variadic, abi }) => {
+                            let (variant, variant_place) = downcast(sym::FnPtr)?;
+                            let fn_ptr_place =
+                                self.project_field(&variant_place, FieldIdx::ZERO)?;
+                            self.write_fn_ptr_type_info(
+                                fn_ptr_place,
+                                binder.skip_binder().output(),
+                                safety.is_unsafe(),
+                                *c_variadic,
+                                (!abi.is_rustic_abi()).then_some(abi.as_str()),
+                                binder.skip_binder().inputs(),
+                            )?;
+                            variant
+                        }
                         ty::Adt(_, _)
                         | ty::Foreign(_)
                         | ty::Pat(_, _)
                         | ty::FnDef(..)
-                        | ty::FnPtr(..)
                         | ty::UnsafeBinder(..)
                         | ty::Closure(..)
                         | ty::CoroutineClosure(..)
@@ -354,6 +415,57 @@ impl<'tcx> InterpCx<'tcx, CompileTimeMachine<'tcx>> {
         interp_ok(())
     }
 
+    pub(crate) fn write_fn_ptr_type_info(
+        &mut self,
+        place: impl Writeable<'tcx, CtfeProvenance> + 'tcx,
+        output: Ty<'tcx>,
+        unsafety: bool,
+        variadic: bool,
+        abi: Option<&'static str>,
+        inputs: &[Ty<'tcx>],
+    ) -> InterpResult<'tcx> {
+        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(unsafety), &field_place)?;
+                }
+                sym::abi => {
+                    if let Some(abi) = abi {
+                        let (variant, variant_place) = self.downcast(&field_place, sym::Named)?;
+                        let str_place = self.allocate_str_dedup(abi)?;
+                        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)?;
+                    } else {
+                        let (rust_variant, _rust_place) = self.downcast(&field_place, sym::Rust)?;
+                        self.write_discriminant(rust_variant, &field_place)?;
+                    }
+                }
+                sym::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 => {
+                    self.write_type_id(output, &field_place)?;
+                }
+                sym::variadic => {
+                    self.write_scalar(Scalar::from_bool(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

@@ -247,6 +247,7 @@ symbols! {
         Fn,
         FnMut,
         FnOnce,
+        FnPtr,
         Formatter,
         Forward,
         From,
@@ -299,6 +300,7 @@ symbols! {
         Mutex,
         MutexGuard,
         N,
+        Named,
         NonNull,
         NonZero,
         None,
@@ -1279,6 +1281,7 @@ symbols! {
         inline_const,
         inline_const_pat,
         inout,
+        inputs,
         instant_now,
         instruction_set,
         integer_: "integer", // underscore to avoid clashing with the function `sym::integer` below
@@ -1647,6 +1650,7 @@ symbols! {
         os_string_as_os_str,
         other,
         out,
+        output,
         overflow_checks,
         overlapping_marker_traits,
         owned_box,
@@ -2427,6 +2431,7 @@ symbols! {
         unsafe_no_drop_flag,
         unsafe_pinned,
         unsafe_unpin,
+        unsafety,
         unsize,
         unsized_const_param_ty,
         unsized_const_params,
@@ -2471,6 +2476,7 @@ symbols! {
         value,
         values,
         var,
+        variadic,
         variant_count,
         vec,
         vec_as_mut_slice,

library/core/src/mem/type_info.rs

@@ -63,6 +63,8 @@ pub enum TypeKind {
     Reference(Reference),
     /// Pointers.
     Pointer(Pointer),
+    /// Function pointers.
+    FnPtr(FnPtr),
     /// FIXME(#146922): add all the common types
     Other,
 }
@@ -202,3 +204,35 @@ 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", "C" etc.
+    Named(&'static str),
+    /// Default
+    #[default]
+    Rust,
+}

library/coretests/tests/lib.rs

@@ -1,6 +1,7 @@
 // tidy-alphabetical-start
 #![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
 #![cfg_attr(test, feature(cfg_select))]
+#![feature(abi_custom)]
 #![feature(array_ptr_get)]
 #![feature(array_try_from_fn)]
 #![feature(array_try_map)]

library/coretests/tests/mem.rs

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