Tracking issue for release notes of #130654: stabilize `-Znext-solver=coherence` again

[rustbot]

This issue tracks the release notes text for #130654.

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).

# Language
- [use the next-generation trait solver in coherence](https://github.com/rust-lang/rust/pull/130654)

Tip

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Release blog section

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### Next-generation trait solver in coherence

Coherence checking, i.e. the part of the compiler making sure that there is no overlap between different trait implementations, the "coherence check", now uses the next-generation trait solver. This changes fixes many subtle inconsistencies, bugs, and even unsoundnesses. TODO links

The next-generation trait solver is a reimplementation of a core component of Rusts type system. It is not only responsible for checking whether trait-bounds - e.g. `Vec<T>: Clone` - hold, but is also used by many other parts of the type system, such as normalization - figuring out the underlying type of `<Vec<T> as IntoIterator>::Item` - and equating types.

TODO @lcnr

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