Rust 2021 will probably be stabilied soon. Cargo also has knowledge of which editions are stable, so should be updated too. It'd be good to prepare that PR, so we can merge it as soon the edition is stabilized in rustc.

Alternatively, can we make it such that Cargo will allow --edition=2021 but will not pass -Z unstable-options when the edition2021 cargo feature is not enabled? Then it'll be usable as soon as rustc allows it, without having to do the cargo change in sync.