Rust: How do you implement iterator patterns in Rust?

1struct Counter {
2    count: u32,
3    max: u32,
4}
5
6impl Iterator for Counter {
7    type Item = u32;
8
9    fn next(&mut self) -> Option<Self::Item> {
10        if self.count < self.max {
11            self.count += 1;
12            Some(self.count)
13        } else {
14            None
15        }
16    }
17}
18
19let counter = Counter { count: 0, max: 5 };
20for i in counter {
21    println!("{}", i);  // 1, 2, 3, 4, 5
22}

1let sum: u32 = (1..=10)
2    .filter(|x| x % 2 == 0)
3    .map(|x| x * x)
4    .sum();