Tracking issue for release notes of #137944: Sized Hierarchy: Part I

[rustbot]

This issue tracks the release notes text for #137944.

cc @davidtwco, @oli-obk, @lcnr -- original issue/PR authors and assignees for drafting text

See the forge.rust-lang.org chapter about release notes for an overview of how the release team makes use of these tracking issues.

Release notes text

This section should be edited to specify the correct category(s) for the change, with succinct description(s) of what changed. Some things worth considering:

• Does this need an additional compat notes section?
• Was this a libs stabilization that should have additional headers to list new APIs under Stabilized APIs and Const Stabilized APIs?

# Compatibility Notes
- [Sized Hierarchy: Part I](https://github.com/rust-lang/rust/pull/137944)
  - Introduces a small breaking change affecting `?Sized` bounds on impls on recursive types which contain associated type projections. It is not expected to affect any existing published crates. Can be fixed by refactoring the involved types or opting into the `sized_hierarchy` unstable feature. See the [FCP report](https://github.com/rust-lang/rust/pull/137944#issuecomment-2912207485) for a code example.

Tip

Use the previous releases for inspiration on how to write the release notes text and which categories to pick.

Release blog section

If this change is notable enough for inclusion in the blog post then this section should be edited to contain a draft for the blog post. Otherwise leave it empty.

Note

If a blog post section is required the release-blog-post label should be added (@rustbot label +release-blog-post) to this issue as otherwise it may be missed by the release team.

[workingjubilee]

This needs to be about the breaking change we're accepting to land this even as an experiment.

We could also promote using the nightly features though.

[davidtwco]

I've written up the Release notes text, with the category "Internal changes", though I wasn't sure because obviously the breaking change is potentially user impacting, so maybe "Language" would have been more relevant - please change it if I've picked the wrong category. Release blog section left blank.

[Noratrieb]

We have the section compat notes exactly for breaking changes