This is an unfortunate situation currently, but better then the alternative. cargo doesn't tie into the rust compiler (just uses it as a program it calls), but

This is an unfortunate situation currently, but better then the alternative. cargo doesn't tie into the rust compiler (just uses it as a program it calls), but it wants to be always available. Because of that, it builds with a known-good version of Rust to make sure `cargo` can be used in the most recent version on all (supported) platforms. This is due to the instability of Rust. Before that, cargo was always broken, if you didn't have the correct version for building it. This will certainly change once Rust stable released. As all things Rust, the whole ecosystem still has a huge label "in construction". The things that are supported and intended to be kept stable are quite stable, though!