This guide is meant to help document how rustc – the Rust compiler – works, as well as to help new contributors get involved in rustc development. It is not meant to replace code documentation – each chapter gives only high-level details, the kinds of things that (ideally) don't change frequently.
There are three parts to this guide. Part 1 contains information that should be useful no matter how you are contributing. Part 2 contains information about how the compiler works. Finally, there are some appendices at the end with useful reference information.
The guide itself is of course open-source as well, and the sources can be found at the GitHub repository. If you find any mistakes in the guide, please file an issue about it, or even better, open a PR with a correction!
You might also find the following sites useful: