Hey, at least it's generic!

generic.rs

pub trait StreamingIterator<'a, T> {
    /// Return either the next item in the sequence, or `None` if all items
    /// have been consumed.
    fn next(&'a mut self) -> Option<T>;

    /// Hey, it compiles.
    fn reduce<S>(&'a mut self, init: S, r: |S,T| -> S) -> S {
        let mut sum = init;
        //streaming_for!(v in self, {
        //    sum = r(sum, v);
        //});
        sum = r(sum, self.next().unwrap());
        sum
    }
}