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

Yes. I am pretty happy with this PR right now and I would have been pretty annoyed if it tried to bundle that change.

rust-rfcbot · 2026-10-21T13:17:49Z

🔔 This is now entering its final comment period, as per the review above. 🔔

scottmcm · 2026-10-21T15:39:30Z

The title made me ponder potential future things like extern "naked" fn, but that would fit in the "not callable" exception, so this seems straight-forwardly-good.

@rfcbot reviewed

tgross35 · 2026-10-21T19:14:11Z

The title made me ponder potential future things like extern "naked" fn, but that would fit in the "not callable" exception, so this seems straight-forwardly-good.

(unsafe extern "custom"? That's coming in 1.100 #158504)

rust-rfcbot · 2026-10-31T13:18:45Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

workingjubilee

I have this weird "I feel like we want more tests" feeling, but we can always benefit from some fuzzing, so that's not different from anything else in the compiler. This seems good. It does strengthen the promise you make inside unsafe extern with safe from "yeah, safe, anyone deliberately calling this is okay" to "No, really, you can call this as an anonymized closure three crates over, hope you were right that it's fine to call this context-free!" but that's consistent with unsafe elsewhere: that code has got some mighty fine print.

View changes since this review

workingjubilee · 2026-10-31T18:06:54Z

I haven't seen any activity from the randomly-assigned reviewer on this PR, and it has received quite a bit of scrutiny, including now my own.

r? @workingjubilee
@bors r=workingjubilee

rust-bors · 2026-10-31T18:06:57Z

📋 This PR cannot be approved because it currently has the following label: needs-reference-pr.

Jules-Bertholet · 2026-10-31T19:08:14Z

Has reference PR: rust-lang/reference#2383

workingjubilee · 2026-11-01T02:14:00Z

This has unanimous approval by lang, FCP has closed, and the reference PR was presumably reviewed (as it was certainly viewable) during discussion of this feature. In the interest of avoiding a "Who's on first?" skit, I'm gonna r+ this again but if I should've waited until the ref PR was formally merged or something please let me know.

@bors r+

rust-bors · 2026-11-01T02:14:04Z

📌 Commit 4621dfe has been approved by workingjubilee

It is now in the queue for this repository.

Rollup of 10 pull requests Successful merges: - #160186 (Implement `Fn` traits for safe functions of any ABI) - #160370 (rustdoc: notable trait badge and popover enhancements) - #165077 (Fix alloctests for panic=abort) - #163623 (Make docblock `<code>` elements inline so they behave better) - #163833 (Fix RISC-V inline ASM `clobber_abi` when extra target features are enabled) - #165067 (cg_llvm: Use an invariant lifetime brand for LLVM modules ) - #165227 (Respect `--color=never` when printing `--explain` output to the terminal) - #165254 (Cleanup `rustdoc-html` tests) - #165258 (impl Eq for AllocRange) - #165259 (rustc-dev-guide subtree update)

@rustbot

Rollup merge of #160186 - Jules-Bertholet:fn-traits-any-abi, r=workingjubilee Implement `Fn` traits for safe functions of any ABI This brings the behavior of the language in line with its documentation: https://github.com/rust-lang/rust/blob/1a833e16546c2eb012758ddd499964fd8afee29e/library/core/src/primitive_docs.rs#L1930-L1931 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

Rollup of 10 pull requests Successful merges: - rust-lang/rust#160186 (Implement `Fn` traits for safe functions of any ABI) - rust-lang/rust#160370 (rustdoc: notable trait badge and popover enhancements) - rust-lang/rust#165077 (Fix alloctests for panic=abort) - rust-lang/rust#163623 (Make docblock `<code>` elements inline so they behave better) - rust-lang/rust#163833 (Fix RISC-V inline ASM `clobber_abi` when extra target features are enabled) - rust-lang/rust#165067 (cg_llvm: Use an invariant lifetime brand for LLVM modules ) - rust-lang/rust#165227 (Respect `--color=never` when printing `--explain` output to the terminal) - rust-lang/rust#165254 (Cleanup `rustdoc-html` tests) - rust-lang/rust#165258 (impl Eq for AllocRange) - rust-lang/rust#165259 (rustc-dev-guide subtree update)

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 Resolved

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 Resolved

Jules-Bertholet commented Jul 30, 2026

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rustbot assigned jieyouxu Aug 24, 2026

rust-rfcbot removed the proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. label Oct 21, 2026

traviscross removed I-lang-nominated Nominated for discussion during a lang team meeting. P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang labels Oct 21, 2026

rustbot mentioned this pull request Oct 31, 2026

Tracking issue for release notes of #160186: Implement Fn traits for safe functions of any ABI #165232

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workingjubilee approved these changes Oct 31, 2026

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rustbot assigned workingjubilee and unassigned TaKO8Ki Oct 31, 2026

workingjubilee removed the needs-reference-pr This language change needs an approved Reference PR to proceed. label Oct 31, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 1, 2026

jhpratt mentioned this pull request Nov 1, 2026

Rollup of 10 pull requests #165264

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rust-bors Bot merged commit dd9fc74 into rust-lang:main Nov 1, 2026
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rustbot added this to the 1.101.0 milestone Nov 1, 2026

Jules-Bertholet deleted the fn-traits-any-abi branch November 1, 2026 23:48

	/// 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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