mrustc doesn't support all of Rust, its only purpose is to bootstrap the normal Rust compiler; and it only works on x86 and x86_64 Linux and isn't very much optimized (eg. needs about 10GB of RAM for the final linking).
And it's lagging behind by a year or two (which is about 15 intermediate releases of the Rust compiler you need to compile to get the current one); because it only has a single developer and keeping up is a huge amount of work.
