Tracking issue for release notes of #129021: Check WF of source type's signature on fn pointer cast

[rustbot]

This issue tracks the release notes text for #129021.

• 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.

# Pick a category...
- [Check WF of source type's signature on fn pointer cast](https://github.com/rust-lang/rust/pull/129021)

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, @lcnr -- origin issue/PR authors and assignees for starting to draft text

[compiler-errors]

Ooh this one will be difficult to write up a description for, though it's worth mentioning perhaps? It does break cve-rs, which is cool. ^{though they could work around it if they tried harder...}

This PR partly closes a soundness hole that comes when casting a function item to function pointer, but I'm not sure how best to describe it otherwise...

I don't expect users to hit this in practice, though, so I'm not totally sure if it needs relnotes...