Function index_set Copy item path

pub exec fn index_set<T, Idx, E>(container: &mut T, index: Idx, val: E)
where
    T: ?Sized + IndexMut<Idx> + Index<Idx, Output = E> + IndexSetTrustedSpec<Idx>,

old(container).spec_index_set_requires(index),

old(container).spec_index_set_ensures(final(container), index, val),