Make exhaustiveness usable outside of rustc With this PR, `rustc_pattern_analysis` compiles on stable (with the `stable` feature)! `rust-analyzer` will be able to use it to provide match-related diagnostics and refactors. Two questions: - Should I name the feature `nightly` instead of `rustc` for consistency with other crates? `rustc` makes more sense imo. - `typed-arena` is an optional dependency but tidy made me add it to the allow-list anyway. Can I avoid that somehow? r? `@compiler-errors`