Macro save_spare 

macro_rules! save_spare {
    ($spare:expr, $orig:expr) => { ... };
}

Save the changes to GenericVec::spare_capacity_mut

$orig - a mutable reference to a GenericVec $spare - the SliceVec that was obtained from [$orig.spare_capacity_mut()]

Safety

$spare should be the SliceVec returned by $orig.spare_capacity_mut()