That's the heart of Mara's Rust proc macro nightly_crimes!

This macro allows you to use Rust's unstable nightly features from your stable Rust programs, in some sense this is clearly silly and can't work, but it actually works fine†. The proc macro is allowed to run software, in this case it destroys the running compiler and launches a different compiler, then it tidies up the resulting mess.

† In a very literal sense, obviously in principle you should never use this, that's why it is named nightly_crimes! not perfectly_reasonable_idea!