Lib.rs

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.