Add a .peekable() to iterators. It returns a new iterators that has an additional .peek() function returning a &ref to the next item.

peekable.rs

trait Peekable<I, T> {
    fn peekable(self) -> PeekingIterator<I, T>;
}

impl<I: Iterator<T>, T> Peekable<I, T> for I {
    fn peekable(self) -> PeekingIterator<I, T> {
        PeekingIterator{ iter: self, peeked: None }
    }
}


struct PeekingIterator<I, T> {
    iter: I,
    peeked: Option<T>,
}

impl<I: Iterator<T>, T> Iterator<T> for PeekingIterator<I, T> {
    fn next(&mut self) -> Option<T> {
        if self.peeked.is_some() { self.peeked.take() }
        else { self.iter.next() }
    }
}

impl<'self, I: Iterator<T>, T> PeekingIterator<I, T> {
    fn peek(&'self mut self) -> Option<&'self T> {
        match self.peeked {
            Some(ref value) => Some(value),
            None => {
                self.peeked = self.iter.next();
                match self.peeked {
                    Some(ref value) => Some(value),
                    None => None,
                }
            },
        }
    }
}