Bootstrap: Build compiler crates that ought to compile on stable without RUSTC_BOOTSTRAP=1

[fmease]

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.

[lukas-code]

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

FWIW, there is already the unstable_features lint. The docs say it's deprecated, but is appears to work perfectly fine.

[bjorn3]

And we have -Zallow-feature to explicitly list the set of allowed unstable features. Could be set in the rustc wrapper of bootstrap depending on the crate name.

[clubby789]

The docs say it's deprecated, but is appears to work perfectly fine

Seems like this note was added in #26417, when use of unstable features was turned into a compiler error rather than a lint

[fmease]

Okay, I guess I'm gonna submit a t-lang-nominated PR later undeprecating unstable_features and applying it to the two affected crates.