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.