Function flat_map 

pub const fn flat_map<Func, In, IntoIter, Meta, Next>(
    func: Func,
    next: Next,
) -> FlatMap<Next, Func, IntoIter, Meta>
where
    Func: FnMut(In) -> IntoIter,
    IntoIter: IntoIterator,
    Meta: Copy,
    Next: Push<IntoIter::Item, Meta>,

Creates a FlatMap push that maps each item to an iterator and flattens the results.