Implement `Fn` traits for safe functions of any ABI by Jules-Bertholet · Pull Request #160186 · rust-lang/rust

Jules-Bertholet · 2026-07-29T23:20:30Z

This brings the behavior of the language in line with its documentation:

Lines 1930 to 1931 in 1a833e1

    
           /// In addition, all *safe* function pointers implement [`Fn`], [`FnMut`], and [`FnOnce`], because 
        
           /// these traits are specially known to the compiler.

ABIs that are not callable, or that are internal implementation details, are excluded.

Reference PR: rust-lang/reference#2383

@rustbot label T-lang needs-fcp A-ABI A-extern-fn I-lang-nominated

rustbot · 2026-07-29T23:20:33Z

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

rustbot · 2026-07-29T23:20:36Z

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ChayimFriedman2 · 2026-07-29T23:43:28Z

Don't you need to generate shims or Happy Things will happen?

Jules-Bertholet · 2026-07-29T23:59:03Z

I don't think so? The basic tests I added work fine, and Miri doesn't complain. But I don't really know that part of the codebase, am relying on Cunningham's Law

ChayimFriedman2 · 2026-07-30T00:00:50Z

I suggest you check with &dyn Fn(), things are more likely to break there.

Jules-Bertholet · 2026-07-30T00:41:23Z

Nothing broke…

ChayimFriedman2 · 2026-07-30T00:42:26Z

🤷 Maybe it is fine...

ChayimFriedman2 · 2026-07-30T00:48:25Z

Although the practical ABI for &isize and isize is identical for C and Rust, so it will compile fine and I don't know if Miri can detect a mismatch - maybe try something like [usize; 2], that appears to have a different ABI between Rust and C?

ds84182 · 2026-07-30T02:10:55Z

Probably should make sure the function that calls the &dyn Fn is not inlined.

RalfJung · 2026-07-30T06:50:06Z

Yeah I'd expect the rust-call / argument (un)tupling to go wrong here as that needs the Rust ABI.

Does this actually solve a problem? If not I'd rather update the docs.

Jules-Bertholet · 2026-07-30T13:00:38Z

Modified the tests to use a function that I confirmed with Godbolt has a completely different "Rust" vs. "C" ABI. And everything still works

Jules-Bertholet · 2026-07-30T13:04:44Z

Does this actually solve a problem?

With new ABIs like "tail", extern is not just for FFI anymore.

RalfJung · 2026-07-30T13:09:28Z

I guess what happens is that we synthesize a call/call_once body for the traits when implementing them for a fn item, which discards the self argument, and which in this case also serves as an ABI adapter?

But someone should dig into where that happens.

Jules-Bertholet · 2026-07-30T14:50:12Z


    pub fn is_fn_trait_compatible(self) -> bool {
-        !self.c_variadic() && self.safety().is_safe() && self.abi() == ExternAbi::Rust
+        !self.c_variadic() && self.safety().is_safe() && self.abi().is_fn_trait_compatible()


Suggested change

!self.c_variadic() && self.safety().is_safe() && self.abi().is_fn_trait_compatible()

self.safety().is_safe() && self.abi().is_fn_trait_compatible()

We could try to remove the !self.c_variadic() condition. But there's not much point, as any actual use of variadics is necessarily unsafe. It would also be a bunch of work, as the trait solver currently assumes no type has more than one impl of each Fn trait.

View changes since the review

teor2345 · 2026-10-11T13:36:23Z

Does this need an update to the Rust Reference, which currently says:

Fn, FnMut, and FnOnce are implemented unless the function has any of the following:
…

an ABI other than "Rust"

https://doc.rust-lang.org/stable/reference/types/function-item.html#r-type.fn-item.traits

Or do we do those kinds of updates later?

Jules-Bertholet · 2026-10-11T23:41:39Z

Reference PR: rust-lang/reference#2383

This brings the behavior of the language in line with the documentation in `library/core/src/primitive_docs.rs`.

rustbot · 2026-10-13T02:31:30Z

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.

tmandry · 2026-10-14T17:50:26Z

@rfcbot fcp merge lang

We discussed this in the lang meeting today, and we were in agreement that this is a positive change. We should do it if we can get away with it (pending crater results).

@rfcbot concern wait for crater results

rust-rfcbot · 2026-10-14T17:50:30Z

@tmandry has proposed to merge this. The next step is review by the rest of the tagged team members:

Concerns:

wait for crater results (Implement Fn traits for safe functions of any ABI #160186 (comment))

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

traviscross · 2026-10-14T17:51:29Z

Makes sense to me. Thanks @Jules-Bertholet (and thanks for filing the Reference PR).

@rfcbot reviewed

joshtriplett · 2026-10-14T17:51:45Z

@rfcbot reviewed

joshtriplett · 2026-10-14T17:52:28Z

I remember we talked about tracking this in some fashion, but I don't think that applies to the Fn traits. We want it for lowercase fn types, to distinguish "this is a pointer to a cdecl function" vs "this is a pointer to a stdcall function". But for capital-F Fn, we only care whether Rust knows how to call it.

lcnr · 2026-10-14T20:59:00Z

I see some discussion about whether this causes ABI compatability problems. Why does it not cause any such issues, can this reasoning be written down somewhere?

Jules-Bertholet · 2026-10-14T21:47:23Z

Why does it not cause any such issues, can this reasoning be written down somewhere?

@teor2345 tracked it down in #160186 (comment). Essentially, nothing in build_call_shim() relies on a specific ABI

RalfJung · 2026-10-15T06:01:20Z

Or put differently, IIUC this ends up generating a shim that serves as an ABI conversion trampoline.

theemathas · 2026-10-18T12:46:04Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-164148-crater-rollup/retry-regressed-list.txt p=1

craterbot · 2026-10-18T12:46:09Z

👌 Experiment pr-160186 created and queued.
🤖 Automatically detected try build 895980e
⚠️ Try build based on commit d9d97c4, but latest commit is 4621dfe. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-10-19T00:56:05Z

🚧 Experiment pr-160186 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-10-19T05:13:46Z

🎉 Experiment pr-160186 is completed!
📊 0 regressed and 0 fixed (28882 total)
📊 1388 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-160186/retry-regressed-list.txt ↩

theemathas · 2026-10-19T05:17:18Z

@tmandry Could you resolve your concern? Thanks.

rustbot assigned jackh726 Jul 29, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Jul 29, 2026

rustbot added A-ABI Area: Concerning the application binary interface (ABI) A-extern-fn Area: `extern` functions needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. T-lang Relevant to the language team labels Jul 29, 2026

RalfJung reviewed Jul 30, 2026

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Comment thread src/tools/miri/tests/pass/fn-traits-abi.rs

Jules-Bertholet force-pushed the fn-traits-any-abi branch from 32a4f77 to 2403142 Compare July 30, 2026 12:59

RalfJung reviewed Jul 30, 2026

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Comment thread compiler/rustc_abi/src/extern_abi.rs

Jules-Bertholet commented Jul 30, 2026

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Oct 11, 2026

theemathas added the needs-reference-pr This language change needs an approved Reference PR to proceed. label Oct 11, 2026

Jules-Bertholet mentioned this pull request Oct 11, 2026

Document Fn traits implemented for all ABIs rust-lang/reference#2383

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Implement Fn traits for functions of any ABI

4621dfe

This brings the behavior of the language in line with the documentation in `library/core/src/primitive_docs.rs`.

Jules-Bertholet force-pushed the fn-traits-any-abi branch from cd9fa00 to 4621dfe Compare October 13, 2026 02:31

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 18, 2026

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Oct 19, 2026

	/// In addition, all safe function pointers implement [`Fn`], [`FnMut`], and [`FnOnce`], because
	/// these traits are specially known to the compiler.

	!self.c_variadic() && self.safety().is_safe() && self.abi().is_fn_trait_compatible()
	self.safety().is_safe() && self.abi().is_fn_trait_compatible()

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