rust 1.54 can be bootstrapped from C++ using https://github.com/thepowersgang/mrustc.
We should update the dev-lang/rust ebuilds such that we can cleanly bootstrap newest rust with mrustc as a base (and clean up ancient versions at the same time).
- Drop <dev-lang/rust-1.54.0 ebuilds
- Add default-enabled mrustc-bootstrap useflag to dev-lang/rust-1.54.0
Switch dev-lang/rust ebuilds from `stable/version` slots to `version` slots so that multiple versions can be installed simultaneously, which seems to be required for portage to successfully resolve the dependencies such that trying to emerge the newest version will build each required earlier version in turn without complaining about a slot conflict
- Remove rust-bin as a possible system-bootstrap dependency (since we autogen the newest version only and older versions are not available for bootstrapping)
I've already successfully written an ebuild that allows bootstrapping dev-lang/rust-1.54.0 with mrustc, I'm now on the last "slot" part, that is trying to recursively build the newest version by automatically building all necessary previous ones.