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Rollup merge of #111087 - ibraheemdev:patch-15, r=dtolnay

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Implement `Sync` for `mpsc::Sender`

`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
This commit is contained in:
Michael Goulet 2023-06-23 19:47:19 -07:00 • committed by GitHub
commit 4a01a38466
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9 changed files with 61 additions and 81 deletions
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4
library/std/src/sync/mpsc/mod.rs
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@ -347,8 +347,8 @@ pub struct Sender<T> {
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send> Send for Sender<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> !Sync for Sender<T> {}
#[stable(feature = "mpsc_sender_sync", since = "CURRENT_RUSTC_VERSION")]
unsafe impl<T: Send> Sync for Sender<T> {}
/// The sending-half of Rust's synchronous [`sync_channel`] type.
///