I asked about that. It doesn't compile Rust, but only the exact subset and dialect of Rust that was used to code exactly one program: the Rust 1.19 compiler.

In other words, it's not properly a compiler, as such, but a bootstrapping tool. It does important compiler-esque things, but not others, and is useful to no one else.

One could argue that’s not a rust compiler because it doesn’t have rust’s most-defining feature: the borrow checker (”mrustc works by compiling assumed-valid rust code (i.e. without borrow checking)”)