Reflection TypeKind::FnPtr by 9SonSteroids · Pull Request #152173 · rust-lang/rust

9SonSteroids · 2026-02-05T14:55:07Z

This is for #146922.

Const-eval currently lacks full support for function pointer (fn) types. We should implement handling of FnPtr TypeKind, covering safe and unsafe functions, Rust and custom ABIs, input and output types, higher-ranked lifetimes, and variadic functions.

rustbot · 2026-02-05T15:23:49Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

The reflection data structures are tied exactly to the implementation
in the compiler. Make sure to also adjust rustc_const_eval/src/const_eval/type_info.rs

cc @oli-obk

rustbot · 2026-02-05T15:23:52Z

r? @nnethercote

rustbot has assigned @nnethercote.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

library/core/src/mem/type_info.rs

compiler/rustc_const_eval/src/const_eval/type_info.rs

rustbot · 2026-02-05T19:17:50Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

library/core/src/mem/type_info.rs

library/coretests/tests/mem/fn_ptr.rs

9SonSteroids · 2026-02-06T18:01:07Z

Now it suggests "did you mean Abi::C" whenever there is a unknown type C... I don't like that.

oli-obk · 2026-02-06T21:54:57Z

To the best of my knowledge this won't happen on stable, as the enum variant is obivously unstable, and the import suggestions respect that

library/core/src/mem/type_info.rs

nnethercote · 2026-02-07T02:38:15Z

I don't know anything about the reflection work, so I will pass this on to @oli-obk. Please reassign/reroll if that's not appropriate! Thanks.

r? @oli-obk

rustbot · 2026-02-07T02:38:21Z

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2026-02-16T13:44:32Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

oli-obk

First round. May result in other cleanups being possible, so maybe check if you can spot sth once you addressed the things I saw

View changes since this review

compiler/rustc_const_eval/src/const_eval/type_info.rs

- 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

oli-obk

ok last round, then it's ready

View changes since this review

oli-obk · 2026-02-17T21:45:31Z

compiler/rustc_const_eval/src/const_eval/type_info.rs

+                                self.project_field(&variant_place, FieldIdx::ZERO)?;
+
+                            // FIXME: handle lifetime bounds
+                            let skipped_binder = sig.skip_binder();


Suggested change

let skipped_binder = sig.skip_binder();

let sig = sig.skip_binder();

skipped_binder does not carry information about what the variable contains

oli-obk · 2026-02-17T21:45:55Z

compiler/rustc_const_eval/src/const_eval/type_info.rs

+
+                            self.write_fn_ptr_type_info(
+                                fn_ptr_place,
+                                skipped_binder.output(),


just pass in the entire sig and take it apart inside

oli-obk · 2026-02-17T21:46:18Z

compiler/rustc_const_eval/src/const_eval/type_info.rs

+        fn_header: &FnHeader<TyCtxt<'tcx>>,
+        inputs: &[Ty<'tcx>],
+    ) -> InterpResult<'tcx> {
+        let FnHeader { safety, c_variadic, abi, .. } = fn_header;


make it exhaustive and document what we are ignoring and not showing reflection users

oli-obk · 2026-02-17T22:00:39Z

library/core/src/mem/type_info.rs

+    ExternRust,
+
+    /// C-calling convention
+    ExternC,


😆 is that Extern part for avoiding the weird diagnostic?

9SonSteroids force-pushed the fn_ptr_type_info branch from 08ac92d to 1a4e30c Compare February 5, 2026 15:23

9SonSteroids marked this pull request as ready for review February 5, 2026 15:23

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 5, 2026

rustbot assigned nnethercote Feb 5, 2026

9SonSteroids marked this pull request as draft February 5, 2026 15:29

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 5, 2026

9SonSteroids force-pushed the fn_ptr_type_info branch from 1a4e30c to 0983ccc Compare February 5, 2026 17:09

9SonSteroids marked this pull request as ready for review February 5, 2026 17:09

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 5, 2026

SpriteOvO added the F-type_info #![feature(type_info)] label Feb 5, 2026