hash_map::IterMut changed Send bounds with hashbrown

[cuviper]

In 1.35, IterMut's Send requires K: Send, but in 1.36-beta it requires K: Sync. Both are auto-derived, so it must be a change in their internals. Obviously, hashbrown is quite different than the old implementation.

https://doc.rust-lang.org/1.35.0/std/collections/hash_map/struct.IterMut.html#impl-Send

https://doc.rust-lang.org/beta/std/collections/hash_map/struct.IterMut.html#impl-Send

Ditto for ValuesMut. I'm not sure if there's any real impact in this, but it should be considered, and perhaps audit for other similar changes.

cc @Amanieu, as discussed on IRC.

[Amanieu]

This is fixed in hashbrown 0.4.0.

[aclonegeek]

@Amanieu Shouldn't ValuesMut be fixed as well?

[cuviper]

Since ValuesMut is actually based on IterMut, it derives the same constraints.
https://docs.rs/hashbrown/0.4.0/hashbrown/hash_map/struct.ValuesMut.html#synthetic-implementations

[aclonegeek]

@cuviper Ah, gotcha. Thanks! Sorry for the false alarm :).