From 998a8777379d8f1cd7b263eff50411ba56f1f9e7 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 29 Apr 2017 09:11:51 +0200
Subject: [PATCH 1/2] MutexGuard<T> may be Sync only if T is Sync

Also remove some unnecessary unsafe impl from the tests.
---
 src/libstd/sync/mutex.rs                 |  8 +++-----
 src/test/compile-fail/mutexguard-sync.rs | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+), 5 deletions(-)
 create mode 100644 src/test/compile-fail/mutexguard-sync.rs

diff --git a/src/libstd/sync/mutex.rs b/src/libstd/sync/mutex.rs
index a27f621e6b2ae..bd5a0fd9828c1 100644
--- a/src/libstd/sync/mutex.rs
+++ b/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 = "rust1", 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(());
diff --git a/src/test/compile-fail/mutexguard-sync.rs b/src/test/compile-fail/mutexguard-sync.rs
new file mode 100644
index 0000000000000..861714720c57a
--- /dev/null
+++ b/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
+}

From 23522f6cabf9fc5803411c3dec4a90c56cc3dab2 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 29 Apr 2017 19:15:59 +0200
Subject: [PATCH 2/2] need to pick a new feature name

---
 src/libstd/sync/mutex.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstd/sync/mutex.rs b/src/libstd/sync/mutex.rs
index bd5a0fd9828c1..5c3eaa4668d28 100644
--- a/src/libstd/sync/mutex.rs
+++ b/src/libstd/sync/mutex.rs
@@ -153,7 +153,7 @@ pub struct MutexGuard<'a, T: ?Sized + 'a> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T: ?Sized> !Send for MutexGuard<'a, T> { }
-#[stable(feature = "rust1", since = "1.18.0")]
+#[stable(feature = "mutexguard", since = "1.18.0")]
 unsafe impl<'a, T: ?Sized + Sync> Sync for MutexGuard<'a, T> { }
 
 impl<T> Mutex<T> {