Well, you have to make sure that you use atomic reference counting in this case. Using regular reference counting will lead to subtle and hard-to-diagnose bugs.

Also, to answer your query, Rust uses unique pointers (the equivalent of std::unique_ptr in C++0x) for inter-thread communication. When the unique pointer goes out of scope, its memory is immediately freed (since it was the only owner of the data). And since the data has only one owner, it's data-race-free.