This (experimental) crate declares a single type FnMutant,
FnMutant has a field f: for<'a> Fn(X) -> Y,
This specifies that the closure must be valid for all lifetimes,
thus any references given to f, through X, must not be captured by the closure.
This results in a function that can be called multiple times (unlike FnOnce),
and does not capture the unique borrow (unlike FnMut).
Because after being called, given an &mut T, the &mut T either been returned or have gone out of scope.