The difference is that memory safety of any kind (including leaking everything) in C/C++ requires discipline, whereas in Rust the compiler is what prevents it. And yes, leaking is not part of that guarantee, because leaks cannot cause corruption or undefined behavior.

With that said, while Rust does not guarantee it, it does have much better automatic memory cleanup compared to C++, because every value has only one owner, and the owner automatically drops/destructs it at the end of its scope.

Getting leaks is possible to do with things like Box::leak, or ref-count cycles, but in practice it tends to be explicit, rather than the programmer forgetting to do something.

But preventing memory leaks is actually impossible. Even with garbage collection, memory leaks are common.