type Meters = u32;
type Kilograms = u32;

let m: Meters = 3;
let k: Kilograms = 3;

assert_eq!(m, k);

trait Iterator {
    // associated type declaration
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

struct Once<T>(Option<T>);

impl<T> Iterator for Once<T> {
    // associated type definition
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        self.0.take()
    }
}