Document the rustc_on_unimplemented attribute.

[mejrs]

Moves/improves rustc_on_unimplemented's docs from the dev guide to the compiler, then in the dev guide adds links to it and other relevant diagnostic/lint attributes

Rendered:

[rustbot]

The rustc-dev-guide subtree was changed. If your future PRs only touch the subtree, consider submitting them directly to rust-lang/rustc-dev-guide, which is where the document is primarily maintained (and has faster CI).

cc @BoxyUwU, @tshepang

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @TaKO8Ki

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[BoxyUwU]

waht's the difference between diagnostic::on_unimplemented and rustc_on_unimplemented?

View changes since this review

[BoxyUwU]

what does doc(attribute do?

[mejrs]

like how we document keywords and builtins, this is what makes the docs show up like that

[BoxyUwU]

r? me
r=me after rebasing :3

[mejrs]

waht's the difference between diagnostic::on_unimplemented and rustc_on_unimplemented?

rustc_on_unimplemented is diagnostic::on_unimplemented, but more, the diagnostic attr only has message/label/note and only some of the formatting options