"Operator-equals" are now implementable - The Edition Guide

# "Operator-equals" are now implementable

The various “operator equals” operators, such as `+=` and `-=`, are implementable via various traits. For example, to implement `+=` on a type of your own:

    use std::ops::AddAssign;

    #[derive(Debug)]
    struct Count {
        value: i32,
    }

    impl AddAssign for Count {
        fn add_assign(&mut self, other: Count) {
            self.value += other.value;
        }
    }

    fn main() {
        let mut c1 = Count { value: 1 };
        let c2 = Count { value: 5 };

        c1 += c2;

        println!("{:?}", c1);
    }

This will print `Count { value: 6 }`.