std::collections::hash_map::{Entry, OccupiedEntry, VacantEntry} are never Send/Sync

[sdleffler]

Currently, the HashMap entry types are never Send or Sync. It seems to me that the Entry types should be Send whenever their key/value types are Send and Sync whenever their key/value types are Sync. Properly implementing Send/Sync for OccupiedEntry and VacantEntry will solve the problem for Entry as well. Is there any known reason why Entry and friends should not be Send or Sync?

[cuviper]

For comparison, btree_map::Entry has both Send and Sync -- manually implemented on NodeRef and then automatically propagating up. The similar low level for HashMap would be the RawBucket.

[nox]

It's not clear to me whether propagating from RawBucket is a good idea, given that if the Send impl of hash_map::Iter is removed, hash_map::Iter would then implement Send where K: Send, V: Send instead of K: Sync, V: Sync.