Function flat_map 

pub fn flat_map<Func, In, IntoIter, Si>(
    func: Func,
    sink: Si,
) -> FlatMap<Si, Func, IntoIter>
where
    Func: FnMut(In) -> IntoIter,
    IntoIter: IntoIterator,
    Si: Sink<IntoIter::Item>,

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