So there's two things to talk about here.

First, the language can't save you from getting the program semantics wrong (e.g. if you never delete an entry from a hashmap even after you're done with it, you're leaking that memory). No language can save you from leaks as a general concept.

Second, Rust makes a very specific promise — freedom from data races. Leaking resources does not actually break that promise, because it doesn't allow you to access that resource any more.