This is a tracking issue for #[cfg(version(..))] (rust-lang/rfcs#2523).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on rustc-guide)
• Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

• What is cfg(version(...)) relative to in terms of nightly compilers?

  We could check against what rustc --version says, e.g. nightly being 1.40.0, beta being 1.39.0, and stable being 1.38.0. We could also have cfg(version(...)) at most be relative to the beta compiler. See the RFC for a longer discussion.

  • Resolved in Tracking issue for RFC 2523, #[cfg(version(..))] #64796 (comment)

• Should we also support version = "..." so that crates having a MSRV below when version(...) was stabilized can use the flag?

• Dependency updates cause language changes (src tarballs are vendored without respecting lockfile #79010)