Default methods

Traits can contain default methods, which can be convenient for the implementor of the trait since fewer methods will need to be implemented. Let's add a toggle() default method in the trait:

trait BitSet {
    fn clear(&mut self, index: usize);
    fn is_set(&self, index: usize) -> bool;
    fn set(&mut self, index: usize);

    fn toggle(&mut self, index: usize) {
        if self.is_set(index) {
            self.clear(index);
        } else {
            self.set(index);
        }
    }
}

Since the new method has a body, we don't need to update our previous implementation. However, we could do it to provide a more efficient implementation, for instance:

impl BitSet for u64 {
    // The other methods are the same as before.

    fn toggle(&mut self, index: usize) {
        *self ^= 1 << index;
    }
}