struct DoubleIfNextOdd<I: Iterator>(std::iter::Peekable<I>);

impl<I: Iterator<Item = i32>> Iterator for DoubleIfNextOdd<I> {
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        let double = match self.0.peek() {
            Some(i) if i % 2 != 0 => true,
            _ => false,
        };
        if double {
            self.0.next().map(|i| i * 2)
        }
        else {
            self.0.next()
        }
    }
}

fn main() {
    for i in DoubleIfNextOdd(vec![1, 2, 3].into_iter().peekable()) {
        println!("{}", i);
    }
}