Uplifted from #117819 (comment).

There exist some compiler crates (e.g., rustc_lexer and rustc_parse_format) which ought not use unstable features since they are used by other tools like rust-analyzer which has a “no nightly” policy. The aforementioned crates do contain comments informing rustc devs about it, they are easy to miss however.

If bootstrap were to compile those crates without RUSTC_BOOTSTRAP=1, the issue of PRs accidentally adding #![feature]s to such crates could be prevented in the future.

Alternative: A rustc-internal or regular lint triggering on #![feature]s.