[dbaupp]

If that's the problem, it's really rather easy to define a wrapper that only clones on an insert. (And, I fundamentally disagree that over-copying this is a pervasive issue; there are a few places (mainly old code) where unnecessary Clone-ing is performed, but this is very much the exception.)

  fn find_or_insert_clone<'a, K: Clone, V>(map: &'a mut HashMap<K,V>,
                                           key: &K, val: V) -> &'a mut V {
      match map.find_mut(key) {
          Some(x) => return x,
          None => {}
      }

      // `key` is definitely missing
      map.find_or_insert(key.clone(), val)
  }