Function flat_map_stream 

pub const fn flat_map_stream<Func, In, St, Meta, Next>(
    func: Func,
    next: Next,
) -> FlatMapStream<Next, Func, St, Meta>
where
    Func: FnMut(In) -> St,
    St: Stream,
    Meta: Copy,
    Next: Push<St::Item, Meta>,

Creates a FlatMapStream push that maps each item to a stream and flattens the results.