Rust is everywhere: https://github.com/pypa/pip/blob/7efc8149162d9ae2f5a60cb346979bc8a6e81700/src/pip/_internal/network/session.py#L179-L192

Wow, all of that just to stick the rustc version in the user-agent

If you have

rustup

but you re-define HOME, which Pex tests do, it turns out you get a

.rustup/

dir created in that re-defined home if you run Pip. Foiled some Pex tests that expected an emtpy custom HOME dir.