rollup merge of rust-lang#20326: rohitjoshi/master

fixes rust-lang#20322

Author: alexcrichton

src/libcollections/ring_buf.rs

@@ -48,6 +48,12 @@ pub struct RingBuf<T> {
     ptr: *mut T
 }
 
+#[stable]
+unsafe impl<T: Send> Send for RingBuf<T> {}
+
+#[stable]
+unsafe impl<T: Sync> Sync for RingBuf<T> {}
+
 #[stable]
 impl<T: Clone> Clone for RingBuf<T> {
     fn clone(&self) -> RingBuf<T> {