Rollup merge of #42397 - sfackler:syncsender-sync, r=alexcrichton

Implement Sync for SyncSender

r? @alexcrichton

Author: frewsxcv

src/libstd/sync/mpsc/mod.rs

@@ -499,8 +499,6 @@ unsafe impl<T: Send> Send for Sender<T> { }
 impl<T> !Sync for Sender<T> { }
 
 /// The sending-half of Rust's synchronous [`sync_channel`] type.
-/// This half can only be owned by one thread, but it can be cloned
-/// to send to other threads.
 ///
 /// Messages can be sent through this channel with [`send`] or [`try_send`].
 ///
@@ -555,9 +553,6 @@ pub struct SyncSender<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Send> Send for SyncSender<T> {}
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T> !Sync for SyncSender<T> {}
-
 /// An error returned from the [`Sender::send`] or [`SyncSender::send`]
 /// function on **channel**s.
 ///

src/test/compile-fail/not-sync.rs

@@ -10,7 +10,7 @@
 
 use std::cell::{Cell, RefCell};
 use std::rc::{Rc, Weak};
-use std::sync::mpsc::{Receiver, Sender, SyncSender};
+use std::sync::mpsc::{Receiver, Sender};
 
 fn test<T: Sync>() {}
 
@@ -29,6 +29,4 @@ fn main() {
     //~^ ERROR `std::sync::mpsc::Receiver<i32>: std::marker::Sync` is not satisfied
     test::<Sender<i32>>();
     //~^ ERROR `std::sync::mpsc::Sender<i32>: std::marker::Sync` is not satisfied
-    test::<SyncSender<i32>>();
-    //~^ ERROR `std::sync::mpsc::SyncSender<i32>: std::marker::Sync` is not satisfied
 }