This doesn't sound like a very good example. Fn inherits FnMut inherits FnOnce, so a library should just ask for the freest one it can support. Usually that just falls out of the meaning of the function. For example, a callback should only be called once, so you ask for an FnOnce, or maybe some function will be called in parallel, so you ask for an Fn. And with Rc, RefCell, Arc, etc. a dynamic approach is also pretty well-supported.