declare C and C-unwind as mutually ABI-compatible

[RalfJung]

We left this conservative in #115476. That means the following code is currently UB:

extern "C-unwind" fn does_not_unwind_but_could() {}

fn main() {
    let f: extern "C-unwind" fn() = does_not_unwind_but_could;
    let f: extern "C" fn() = unsafe { std::mem::transmute(f) };
    f();
}

I think this code should be allowed. Zulip discussion also led to the conclusion that this should be fine in LLVM -- the nounwind attribute does not affect the ABI. This is widely relied upon in the C ecosystem when doing calls between C and C++.
Cc @nikic to confirm.

I am not sure why #115476 left this conservative. There was discussion about explicitly listing the ABIs rather than saying this holds universally for all *-unwind that we may add in the future, but there was no discussion I could find about allowing a mismatch both ways.

Cc @rust-lang/opsem @rust-lang/lang

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
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

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from JohnTitor, Mark-Simulacrum, clarfonthey, nia-e

[rustbot]

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.

[RalfJung]

I guess we should FCP this? Not sure if lang wants to be involved. I'll start with opsem.
@nikic @bjorn3 if you have any concerns please let us know!

@rfcbot fcp merge

[RalfJung]

@rfcbot fcp merge opsem

is what I meant...

[rust-rfcbot]

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

• @CAD97
• @RalfJung
• @digama0
• @joshlf
• @saethlin

No concerns currently listed.

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.

[bjorn3]

I don't have any concerns with this either. For a long time extern "C" was effectively what we now call extern "C-unwind" and I don't recall anyone filling an issue about this causing ABI problems at all.

[rust-rfcbot]

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

[traviscross]

We talked about this briefly in the lang meeting today, and all present agreed this made sense.

[rust-rfcbot]

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.

[RalfJung]

@Mark-Simulacrum (or anyone reading along) this now just needs a review. :)

[Mark-Simulacrum]

@bors r+ rollup

[rust-bors]

📌 Commit aee14eb has been approved by Mark-Simulacrum

It is now in the queue for this repository.