Rust中的迭代器

和闭包一样，练代码

struct Counter {
    count: u32,
}

impl Counter {
    fn new() -> Counter {
        Counter {count:  }
    }
}

impl Iterator for Counter {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        self.count += ;

         {
            Some(self.count)
        } else {
            None
        }
    }
}

#[test]
fn using_other_iterator_trait_methods() {
    let sum: u32 = Counter::))
                    .map(|(a, b)| a * b)
                    .filter(|x| x %  == )
                    .sum();
    assert_eq!(, sum);
}