ruex VS rust-design-pattern

here's the observer pattern implemented in rust. Though I understand your point. In general though, if a thing is valid and the borrow checker gets in the way, that's not the language team telling you not to do that thing, that's a limitation of the borrow checker. The big thing that comes into play are reference cycles, which sometimes can be valid, but stops the borrow checker from being able to guarantee ownership. So in situations where you truly can't do something in normal rust because of these limitations, but you know is safe you use unsafe. Rust themselves explain that better than I can: