Function const_size_flat_map 

pub fn const_size_flat_map<I, U, F>(
    iter: I,
    f: F,
) -> ConstSizeFlatMap<I::IntoIter, U, F> 
where
    I: IntoIterator,
    U: IntoIterator,
    F: FnMut(I::Item) -> U,

Construct a ConstSizeFlatMap from an IntoIterator (which includes Iterators).