Add get_mut() to Mutex and RwLock (#322)

Add the standard library get_mut() method to Mutex and RwLock. This
provides mutable access without locking for callers who already hold a
&mut Mutex or &mut RwLock.

src/sync/mutex.rs

@@ -58,6 +58,11 @@ impl<T> Mutex<T> {
         }
     }
 
+    /// Returns a mutable reference to the underlying data.
+    pub fn get_mut(&mut self) -> LockResult<&mut T> {
+        Ok(self.data.get_mut().unwrap())
+    }
+
     /// Consumes this mutex, returning the underlying data.
     pub fn into_inner(self) -> LockResult<T> {
         Ok(self.data.into_inner().unwrap())

src/sync/rwlock.rs

@@ -106,6 +106,11 @@ impl<T> RwLock<T> {
         }
     }
 
+    /// Returns a mutable reference to the underlying data.
+    pub fn get_mut(&mut self) -> LockResult<&mut T> {
+        Ok(self.data.get_mut().expect("loom::RwLock state corrupt"))
+    }
+
     /// Consumes this `RwLock`, returning the underlying data.
     pub fn into_inner(self) -> LockResult<T> {
         Ok(self.data.into_inner().expect("loom::RwLock state corrupt"))