HashMap.mangle no longer exists

[huonw]

There's now no way to be able to conditionally execute expensive functionality when inserting both when the key is not found, and when the key is found, without hashing twice.

See the 0.9 docs for signature.

cc @cgaebel, was this removed on purpose?

[huonw]

(mangle was also useful as a building block for essentially all the other insertion methods, see the 0.9/pre-robin-hood-hashmap source for how it was used.)

[cgaebel]

This was not removed on purpose. However, its benefits are only that you save a rehashing, as with robin hood hashing you must know whether you are inserting or looking up when you probe the table, to know whether or not you need to steal buckets.

[cgaebel]

I feel like there's room for a more usable and powerful abstraction here. Something like:

pub struct HashWithKey<K, 'a> {
  priv hash: SafeHash;
  priv key: &'a K;
}

impl<K, 'a> HashWithKey<K, 'a> {
  pub fn key(&self) -> &'a K;
  fn hash(&self) -> &'a SafeHash;
}

pub fn hash<'a>(&self, k: &'a K) -> HashWithKey<K, 'a>

And expose all my methods that take a prehashed key.

[bluss]

Fixed by #14196

[alexcrichton]

Hurray!