with_next.rs

// example usable
for (child, next) in values.iter().with_next() {
  // do stuff
}   

struct WithNext<I>
where
    I: Iterator,
{
    underlying: Peekable<I>,
}

impl<I> Iterator for WithNext<I>
where
    I: Iterator,
    I::Item: Eq + Copy,
{
    type Item = (I::Item, Option<&I::Item>);

    fn next(&mut self) -> Option<Self::Item> {
        if let Some(next) = self.underlying.next() {
            Some((next, self.underlying.peek()))
        } else {
            None
        }
    }
}

trait WithNextExt: Iterator {
    fn with_next(self) -> WithNext<Self>
    where
        Self::Item: Eq + Clone,
        Self: Sized,
    {
        WithNext {
            underlying: self.peekable(),
        }
    }
}

impl<I: Iterator> WithNextExt for I {}