It has been a long-standing tradition to develop a language far enough to be able to write the language's compiler in the same language, and Rust does the same.
We've tracked down the earlier Rust versions, which were written in OCaml, and were planning to use these to bootstrap Rust.
But in parallel, John Hudge (Mutabah) developed a Rust compiler, called "mrustc", written in C++.
Rust takes reproducible builds seriously, but there are some reproducibility problems left in earlier compilers that pop up very sporadically (mostly because of LLVM, and some because of Rust hashtable poisoning).
In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection.
Guix uses low-level mechanisms from the Nix package manager, except that packages are defined as native Guile modules, using extensions to the Scheme language.