Implement Sync for mpsc::Sender by ibraheemdev · Pull Request #111087 · rust-lang/rust (original) (raw)

mpsc::Sender is currently !Sync because the previous implementation contained an optimization where the channel started out as single-producer and was dynamically upgraded on the first clone, which relied on a unique reference to the sender. This optimization is one of the main reasons the old implementation was so complex and was removed in #93563. mpsc::Sender can now soundly implement Sync.

Note for any potential confusion, this chance does not add MPMC behavior. This only affects the already Send + Clone sender, not receiver.

It's technically possible to rely on the !Sync behavior in the same way as a PhantomData<*mut T>, but that seems very unlikely in practice. Either way, this change is insta-stable and needs an FCP.

@rustbot label +T-libs-api -T-libs