pub trait HashBuilder<T> {
  fn build(key: T) -> usize;
}

struct HashMap<K, V> {
    key: K,
    value: V
}

impl<K, V> HashBuilder<K> for HashMap<K, V> {
    fn build(key: K) -> usize {
        // Some Implementation that changes, respective to the way HashMap key: type is
        // defined...
        return 23 as usize;
    }
}