Function basic_combinations 

pub fn basic_combinations<I>(
    it: I,
    k: usize,
) -> impl Iterator<Item = Box<[I::Item]>>
where
    I: Iterator + Clone,
    I::Item: Clone,

Returns an iterator over all size-k subsets of the elements returned by the given iterator.

This is a special version of combinations(), which accepts a converter function and thus can avoid cloning each subset.