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

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[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. 🔔