rustfmt is a reformatting tool for Rust code. The idea is that it takes your code, tidies it up, and makes sure it conforms to a set of style guidelines. There are similar tools for C++ (clang format), Go (gofmt), and many other languages. Its a really useful tool to have for a language, since it makes it easy to adhere to style guidelines and allows for mass changes when guidelines change, thus making it possible to actually change the guidelines as needed.
Eventually I would like rustfmt to do lots of cool stuff like changing glob imports to list imports, or emit refactoring scripts to rename variables to adhere to naming conventions. In the meantime, there are lots of interesting questions about how to lay out things like function declarations and match expressions.
My approach to rustfmt is very incremental. It is usable now and gives good results, but it only touches a tiny subset of language items, for example function definitions and calls, and string literals. It preserves code elsewhere. This makes it immediately useful.
I have managed to run it on several crates (or parts of crates) in the rust distro. It also bootstraps, i.e., you can rustfmt on rustfmt before every check-in, in fact this is part of the test suite.
It would be really useful to have people running this tool on their own code or on other crates in the rust distro, and filing issues and/or test cases where things go wrong. This should actually be a useful tool to run, not just a chore, and will get more useful with time.
It's a great project to hack on - you'll learn a fair bit about the Rust compiler's frontend and get a great understanding of more corners of the language than you'll ever want to know about. It's early days too, so there is plenty of scope for having a big impact on the project. I find it a lot of fun too! Just please forgive some of the hackey code that I've already written.
Here is the rustfmt repo on GitHub. I just added a bunch of information to the repo readme which should help new contributors. Please let me know if there is other information that should go in there. I've also created some good issues for new contributors. If you'd like to help out and need help, please ping me on irc (I'm nrc).