Function opt_clone_from_or_clone

fn opt_clone_from_or_clone<'a, T: Clone>(
    opt: &'a mut Option<T>,
    val: &T,
) -> &'a mut T

An analogue of Option::get_or_insert_with that stores a clone of val into opt, but uses the more efficient clone_from if opt was Some.

Returns a mutable reference to the new clone that resides in opt.