Like C was used to write C and OCaml was used to write OCaml, Rust is written in Rust. Mostly, anyway, the GitHub repository seems to have also JavaScript, Pyth

Like C was used to write C and OCaml was used to write OCaml, Rust is written in Rust. Mostly, anyway, the GitHub repository seems to have also JavaScript, Python and Shell. Though to actually get binaries out of it LLVM is used, which itself is written in C++.