Why is `RUSTC_BOOTSTRAP` so actively discouraged?

While it hasn't seen much recent motion, there is an RFC that proposes both breaking down RUSTC_BOOTSTRAP into finer grained control, and discouraging that replacement feature less:

github.com/rust-lang/rfcs

RUSTC_ALLOW_UNSTABLE_<feature>: a RUSTC_BOOTSTRAP alternative (#3882)

master ← jyn514:rustc-bootstrap

opened 02:46AM - 22 Nov 25 UTC

        jyn514
      

+187 -0

This RFC was in large part drafted in May 2025, shortly after my Rust All Hands …talk, "Futile Feature Gates". There were about 25 people in that room; I want to thank all of them for their feedback during that session, and everyone who reviewed early drafts of this RFC. Rendered > [!IMPORTANT] > When responding to RFCs, try to use inline review comments (it is possible to leave an inline review comment for the entire file at the top) instead of direct comments for normal comments and keep normal comments for procedural matters like starting FCPs. > > This keeps the discussion more organized.