Explicitly opt out of Sync for cell and mpsc types

These types were already `!Sync`, but this improves error messages when
they are used in contexts that require `Sync`, aligning them with
conventions used with `Rc`, among others.

Author: apasel422

src/libcore/cell.rs

@@ -241,6 +241,9 @@ impl<T:Copy> Cell<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T> Send for Cell<T> where T: Send {}
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> !Sync for Cell<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T:Copy> Clone for Cell<T> {
     #[inline]
@@ -461,6 +464,9 @@ impl<T: ?Sized> RefCell<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> !Sync for RefCell<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone> Clone for RefCell<T> {
     #[inline]

src/libstd/sync/mpsc/mod.rs

@@ -299,6 +299,9 @@ pub struct Receiver<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Send> Send for Receiver<T> { }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> !Sync for Receiver<T> { }
+
 /// An iterator over messages on a receiver, this iterator will block
 /// whenever `next` is called, waiting for a new message, and `None` will be
 /// returned when the corresponding channel has hung up.
@@ -327,6 +330,9 @@ 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> { }
+
 /// The sending-half of Rust's synchronous channel type. This half can only be
 /// owned by one thread, but it can be cloned to send to other threads.
 #[stable(feature = "rust1", since = "1.0.0")]

src/test/compile-fail/comm-not-freeze-receiver.rs

@@ -0,0 +1,34 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::cell::{Cell, RefCell};
+use std::rc::{Rc, Weak};
+use std::sync::mpsc::{Receiver, Sender, SyncSender};
+
+fn test<T: Sync>() {}
+
+fn main() {
+    test::<Cell<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `core::cell::Cell<i32>`
+    test::<RefCell<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `core::cell::RefCell<i32>`
+
+    test::<Rc<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `alloc::rc::Rc<i32>`
+    test::<Weak<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `alloc::rc::Weak<i32>`
+
+    test::<Receiver<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `std::sync::mpsc::Receiver<i32>`
+    test::<Sender<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `std::sync::mpsc::Sender<i32>`
+    test::<SyncSender<i32>>();
+    //~^ ERROR marker::Sync` is not implemented for the type `std::sync::mpsc::SyncSender<i32>`
+}