Implement unstable trait impl

[tiif]

This PR allows marking impls of stable trait with stable type as unstable.

The design and mentoring are done by @BoxyUwU

[bors]

☔ The latest upstream changes (presumably #142299) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #143091) made this pull request unmergeable. Please resolve the merge conflicts.

[lcnr]

pls add PR description providing an outline of the approach

apart from that, a few minor nits, after this r=me :3

Thank you for working on this ❤️

[lcnr]

future work: we likely want an #[rustc_allow_internal_unstable(foo)] attribute which only adds UnstableFeature clauses to the param_env, but not predicates_of.

[lcnr]

I would try to mirror the existing errors for unstable library features if staged_api is not enabled

error[E0658]: use of unstable library feature `slice_partition_dedup`
 --> src/main.rs:3:7
  |
3 |     x.partition_dedup();
  |       ^^^^^^^^^^^^^^^
  |
  = note: see issue #54279 <https://github.com/rust-lang/rust/issues/54279> for more information
  = help: add `#![feature(slice_partition_dedup)]` to the crate attributes to enable
  = note: this compiler was built on 2025-07-02; consider upgrading it if it is out of date

not sure whehter you could even reuse the reporting machinery we already use for methods etc

[lcnr]

Suggested change

	use unstable_feature::{Foo, Bar, Moo};
	use unstable_feature::{Bar, Moo};

[lcnr]

Suggested change

	use unstable_feature::{Foo, Bar};
	use unstable_feature::Bar;

[lcnr]

a few more nits

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.