impl Default for CondVar, Mutex and RwLock

src/libstd/sync/condvar.rs

@@ -227,6 +227,15 @@ impl Drop for Condvar {
     }
 }
 
+#[unstable(feature = "defaults",
+           reason = "has not yet been decided",
+           issue = "31865")]
+impl Default for Condvar {
+    fn default() -> Condvar {
+        Condvar::new()
+    }
+}
+
 impl StaticCondvar {
     /// Creates a new condition variable
     #[unstable(feature = "static_condvar",

src/libstd/sync/mutex.rs

@@ -197,6 +197,15 @@ impl<T> Mutex<T> {
     }
 }
 
+#[unstable(feature = "defaults",
+           reason = "has not yet been decided",
+           issue = "31865")]
+impl<T: Default> Default for Mutex<T> {
+    fn default() -> Mutex<T> {
+        Mutex::new(Default::default())
+    }
+}
+
 impl<T: ?Sized> Mutex<T> {
     /// Acquires a mutex, blocking the current thread until it is able to do so.
     ///

src/libstd/sync/rwlock.rs

@@ -156,6 +156,15 @@ impl<T> RwLock<T> {
     }
 }
 
+#[unstable(feature = "defaults",
+           reason = "has not yet been decided",
+           issue = "31865")]
+impl<T: Default> Default for RwLock<T> {
+    fn default() -> RwLock<T> {
+        RwLock::new(Default::default())
+    }
+}
+
 impl<T: ?Sized> RwLock<T> {
     /// Locks this rwlock with shared read access, blocking the current thread
     /// until it can be acquired.