The personal project I'm currently working on the most is Nucom, an implementation of Microsoft's Component Object Model.

The IDL compiler for it, which takes .idl files which define COM interfaces and outputs C/C++ headers/source files and Rust modules, is written in Rust.

Originally, this project was all C++, and everything but the compiler still is (which also likely isn't going to change since it involves building dynamic libraries which Rust does not do well at all), but I really did not want to go without Rust when writing a parser/compiler type thing because the language is so much nicer to work with.