Tracking issue for release notes of #137037: add x86-sse2 (32bit) ABI that requires SSE2 target feature

[rustbot]

This issue tracks the release notes text for #137037.

Steps

• Proposed text is drafted by PR author (or team) making the noteworthy change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text

The responsible team for the underlying change should edit this section to replace the automatically generated link with a succinct description of what changed, drawing upon text proposed by the author (either in discussion or through direct editing).

# Compatibility Notes
- [SSE2 is now required for "i686" 32-bit x86 hard-float targets; disabling it causes a warning that will become a hard error eventually.](https://github.com/rust-lang/rust/pull/137037) Use an "i586" target to build code for machines without SSE2.

Tip

Use the previous releases categories to help choose which one(s) to use.
The category will be de-duplicated with all the other ones by the release team.

More than one section can be included if needed.

Release blog section

If the change is notable enough for inclusion in the blog post, the responsible team should add content to this section.
Otherwise leave it empty.

cc @RalfJung, @workingjubilee -- origin issue/PR authors and assignees for starting to draft text

[workingjubilee]

I think this is a compatibility note, so edited it in. This is the big one with breaking changes of some nature, effectively: some x86 targets are required to have SSE2 now. Others don't require it and are expected to use a soft-float ABI where possible.

[RalfJung]

Others don't require it and are expected to use a soft-float ABI where possible.

Hm, not sure if the "Rust" ABI is truly equivalent to the softfloat ABI. For "C" functions, it definitely is not -- i586 still uses a hardfloat ABI there.

I would just say, use an i586 target if you need to build code that works on systems without SSE2. Those are tier 2 targets, which reflects that this configuration is not nearly as well-supported as the i686 targets with SSE2.

[riking]

Agreed, the release note should explicitly mention "use a i586 target".