You can't use this implementation to bootstrap Rust (in the sense of bootstrapping from non-Rust language or a compiler that isn't the rustc).

This GCC support here is only a backend in the existing Rust compiler written in Rust. The existing Rust compiler is using GCC as a language-agnostic assembler and optimizer, not as a Rust compiler. The GCC part doesn't even know what Rust code looks like.

There is a different project meant to reimplement Rust (front end) from scratch in C++ in GCC itself, but that implementation is far behind and can't compile non-toy programs yet.