maybe expose a way to pass pre-calculated hashes along with hash table keys

[thestinger]

I found this was necessarily to avoid double-hashing with a shard-based concurrent hash table like Java's ConcurrentHashMap as the hash needs to be calculated to pick the correct shard.

https://github.com/thestinger/rust-core/blob/master/core/concurrent.rs#L299

This could be done by hoisting out the open-addressing implementation, but it will result in it requiring the hashes to be stored in the table rather than it just being an implementation detail.

[cgaebel]

It'd probably need an unsafe tag, and a debug assertation from #12108, but given those I don't see why the HashMap can't support unsafe pub fn insert_hashed(hash: u64, k: K, v: V)

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#794