Tracking issue for RFC 2574, "SIMD vectors in FFI"

[Centril]

This is a tracking issue for the RFC "SIMD vectors in FFI" (rust-lang/rfcs#2574).
The feature gate is #![feature(simd_ffi)].

Steps:

• Implement the RFC (cc @rkruppe @gnzlbg); WIP in [WIP] Implement RFC2574 for FFI declarations #59238 and Implement RFC 2574, "SIMD vectors in FFI" #86546.
• Adjust documentation (see instructions on rustc-guide)
• Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

• Should it be possible to use, e.g., __m128 on C FFI when the avx feature is enabled? Does that change the calling convention and make doing so unsafe ? We could extend this RFC to also require that to use certain types certain features must be disabled.

[RalfJung]

I think this is mostly implemented by the lint tracked at #116558. However, the simd_ffi feature still exists, and if you don't use it you get some warnings or so? I've seen the improper C types warning, but there's a dedicated warning emitted here that I don't think I have ever seen...

Cc @workingjubilee @veluca93

[RalfJung]

Ah, you only get that error when importing a foreign function in an extern "C" block.

I think that can be removed? Calling that function without having the feature gate required for this vector type will lead to an error (well, a future-compat warning for now, an error eventually). And the error tracked here is quite incomplete anyway (see #87438).

We can probably also remove this warning saying that __m256 is not FFI safe when defining an extern "C" function.