Tracking issue for release notes of #129422: Gate repr(Rust) correctly on non-ADT items

[rustbot]

This issue tracks the release notes text for #129422.

• 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). If the change is notable
enough for inclusion in the blog post, add content to the blog section below.

# Compatibility Notes
- [Rust will now correctly raise an error for `repr(Rust)` written on non-`struct`/`enum`/`union` items, since it previous did not have any effect](https://github.com/rust-lang/rust/pull/129422)

Note: The section title will be de-duplicated by the release team with other release notes issues.
Please use a standard title from previous releases.
More than one section can be included if needed.

Release blog section (if any, leave blank if no section is expected):

cc @compiler-errors, @fmease -- origin issue/PR authors and assignees for starting to draft text

[fmease]

T-release, I've tentatively nominated that PR for relnotes/compat/minor_breaking_change. If you think that's overkill, feel free to remove the nomination and close this issue :)

[fmease]

Instead of Gate repr(Rust) correctly on non-ADT items we might want to go for Reject repr(Rust) on items that aren't structs/enums/unions in RELEASES.md which should be easier for laypeople to understand.

[compiler-errors]

Yeah, "Rust will now correctly raise an error for repr(Rust) written on non-struct/enum/union items, since it previous did not have any effect" or something.