Auto merge of #41624 - RalfJung:mutexguard-sync, r=alexcrichton

MutexGuard<T> may be Sync only if T is Sync

Fixes #41622

This is a breaking change. Does that imply any further process?

I am not sure whether I wrote that "compilation must fail"-test correctly, but at least it is passing here with the patch applied. Same for the `since = "1.18.0"`, I just picked it because I had to pick something.

src/libstd/sync/mutex.rs

@@ -10,7 +10,6 @@
 
 use cell::UnsafeCell;
 use fmt;
-use marker;
 use mem;
 use ops::{Deref, DerefMut};
 use ptr;
@@ -153,7 +152,9 @@ pub struct MutexGuard<'a, T: ?Sized + 'a> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T: ?Sized> !marker::Send for MutexGuard<'a, T> {}
+impl<'a, T: ?Sized> !Send for MutexGuard<'a, T> { }
+#[stable(feature = "mutexguard", since = "1.18.0")]
+unsafe impl<'a, T: ?Sized + Sync> Sync for MutexGuard<'a, T> { }
 
 impl<T> Mutex<T> {
     /// Creates a new mutex in an unlocked state ready for use.
@@ -459,9 +460,6 @@ mod tests {
     #[derive(Eq, PartialEq, Debug)]
     struct NonCopy(i32);
 
-    unsafe impl<T: Send> Send for Packet<T> {}
-    unsafe impl<T> Sync for Packet<T> {}
-
     #[test]
     fn smoke() {
         let m = Mutex::new(());

src/test/compile-fail/mutexguard-sync.rs

@@ -0,0 +1,22 @@
+// Copyright 2017 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.
+
+// MutexGuard<Cell<i32>> must not be Sync, that would be unsound.
+use std::sync::Mutex;
+use std::cell::Cell;
+
+fn test_sync<T: Sync>(_t: T) {}
+
+fn main()
+{
+    let m = Mutex::new(Cell::new(0i32));
+    let guard = m.lock().unwrap();
+    test_sync(guard); //~ ERROR the trait bound
+}