What do you mean by "rules out data races"? If I have one piece of code that writes two variables and another that reads them on another thread, there's no way for the compiler to know whether those two changes need to be atomic, serialized, or uncoordinated.

Access to shared mutable memory without use of a synchronization primitive. It's the same definition data race detectors use.

I believe the "data race" definition pcwalton is using is the one from C++11: "a race condition between non-atomic variables". http://en.wikipedia.org/wiki/Race_condition#C.2B.2B

2 threads can't access the same variable in Rust without unsafe{}. I think, anyway...