That's my guess, too. I thought Rust might handle a build system by now with all the libraries pouring out. So, I figured I'd ask anyway in case any interesting answers showed up.

It is a build system written in Rust: https://github.com/rust-lang/rust/tree/master/src/bootstrap

There is only one Python file: https://github.com/rust-lang/rust/blob/master/src/bootstrap/...

Its job is to download the previous Rust release (beta for master, stable for beta and previous stable for current stable), including Cargo, build the build system with Cargo and start it.

The build system then builds native dependencies (LLVM) and runs Cargo several times for different components.

Well, there we go. The other comments made me think it was mostly Python or something.