Tracking Issue for result_ffi_guarantees (RFC 3391)

[tmandry]

This is a tracking issue for the RFC "result_ffi_guarantees" (rust-lang/rfcs#3391).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement (test) the RFC
  • Support Result<T, E> across FFI when niche optimization can be used #122253
  • Support Result<T, E> across FFI when niche optimization can be used (v2) #124747
  • Update Result docs to the new guarantees #124870
  • Add more ABI test cases to miri (RFC 3391) #125672
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide) (not necessary, reference FCP should be all that's needed)

Unresolved Questions

None

Implementation history

[RalfJung]

I did a bunch of work on ABI tests for repr(transparent) that could also be useful for testing this RFC:

• Miri now has this test: https://github.com/rust-lang/miri/blob/master/tests/pass/function_calls/abi_compat.rs
• and rustct will hopefully soon have this: add rustc_abi(assert_eq) to test some guaranteed or at least highly expected ABI compatibility guarantees #115372

[MasterAwesome]

Is anyone still working on this? If not, I'd love to try my shot at it.

[Lokathor]

I have not heard word of anyone working on this in a very long time. It's probably a safe bet to begin a PR.

[RalfJung]

Besides adjusting the improper-ctypes lint (which is happening in #122253), some other things that should be done:

• For Option we have this documentation. I assume we want the same for Result? Is there some way to not repeat the table?
• Miri has a test here that currently only checks Option, not Result.

[MasterAwesome]

Should the documentation change be a separate PR?

[RalfJung]

Yes, that probably makes review easier.