Fn traits

The three traits that say how often a closure can be called and what it may do to its captures.

FnOnce closures can be called at least once, FnMut ones repeatedly with mutable access to captures, Fn ones repeatedly through a shared reference; every Fn is FnMut, and every FnMut is FnOnce.

Examples

fn run_twice<F: Fn()>(task: F) rejects || laps += 1 with E0525: that closure changes a capture, so it is only FnMut.
A move closure whose body does let sent = receipt; is only FnOnce, so calling it twice fails with E0382.