Rust: How do you implement iterator combinators?

1let numbers = vec![1, 2, 3, 4, 5];
2
3// Map
4let doubled: Vec<i32> = numbers.iter().map(|x| x * 2).collect();
5
6// Filter
7let evens: Vec<&i32> = numbers.iter().filter(|x| *x % 2 == 0).collect();
8
9// Fold
10let sum: i32 = numbers.iter().fold(0, |acc, x| acc + x);

1let result: Vec<i32> = (1..=10)
2    .filter(|x| x % 2 == 0)
3    .map(|x| x * x)
4    .collect();

1trait IteratorExt: Iterator {
2    fn unique(self) -> Unique<Self>
3    where
4        Self: Sized,
5        Self::Item: Eq + Hash,
6    {
7        Unique { iter: self, seen: HashSet::new() }
8    }
9}