Rust has &mut references and & references. The `&mut Database` sort of implies that the Database handle doesn't do its own locking or other needed state tracki

Rust has &mut references and & references. The `&mut Database` sort of implies that the Database handle doesn't do its own locking or other needed state tracking - it just relies on the exclusivity of `&mut` in Rust to be sure that nobody else is modifying the database handle at the same time. If the API used `&Database`, then it would have to be more flexible for simultaneous use of the handle. And then the closure would be (a bit) easier to call too, because it's not tracking that exclusive reference. If the API used a reference counted `Database` handle, then it would release even more restrictions of lifetime management, etc. You can implement all of these in Rust. In other languages, you usually end up with alternative two or three. If you try to simultaneously take alternative 1 and use it in a way that's not supported, that would be shooting yourself in the foot. Except Rust says compile error instead.