Add try_write to RwLock

src/librustc_data_structures/sync.rs

@@ -388,6 +388,18 @@ pub fn with_read_lock<F: FnOnce(&T) -> R, R>(&self, f: F) -> R {
         f(&*self.read())
     }
 
+    #[cfg(not(parallel_queries))]
+    #[inline(always)]
+    pub fn try_write(&self) -> Result<WriteGuard<T>, ()> {
+        self.0.try_borrow_mut().map_err(|_| ())
+    }
+
+    #[cfg(parallel_queries)]
+    #[inline(always)]
+    pub fn try_write(&self) -> Result<WriteGuard<T>, ()> {
+        self.0.try_write().ok_or(())
+    }
+
     #[cfg(not(parallel_queries))]
     #[inline(always)]
     pub fn write(&self) -> WriteGuard<T> {