Unfortunately, if you're not a compiler nerd, you may not know what that means.
At the highest level of detail, implementations of a language's grammar are useful even outside of the compiler itself. For example, syntax highlighting tools, static analysis tools, and alternate language implementations are all good usages of a grammar that's separate from the official language implementation. So, this project should help out anyone wanting to do something along those lines.
More specifically, ANTLR is a tool for writing grammars. You can write an ANTLR grammar, then have it generate a bunch of code that you can compile into a project, or use standalone as a grammar checker.
Anyway, as the README shows, it's pretty easy to get started:
1. install antlr 4 jar, put it in your classpath 2. run make 3. try java org.antlr.v4.runtime.misc.TestRig Rust tts -tree < foo.rs
That's it! I'm excited to see some tooling pop up around Rust, and hopefully this will enable even more neat tools to be built.