> Rust's definition of "unsafe" is basically just "whatever Rust prevents".

It's not that circular.

Rust defines data races as unsafe because they can lead to reads that produce corrupt values, outside the set of possibilities defined by their type. It defines memory leaks as safe because they cannot lead to this situation.

That is the yardstick for what makes something safe or unsafe. It is the same yardstick used by other memory-safe languages- for instance, despite your claims to the contrary, garbage collectors do not and cannot guarantee a total lack of garbage. They have a lot of slack to let the garbage build up and then collect it all at once, or in some situations never collect it at all.

There are plenty of undesirable behaviors that fall outside of this definition of unsafety. Memory leaks are simply one example.