diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index a408bf8e284e7..546e4e5269979 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -160,7 +160,8 @@ impl<T: Send + Share + Clone> Arc<T> {
         // reference count is guaranteed to be 1 at this point, and we required
         // the Arc itself to be `mut`, so we're returning the only possible
         // reference to the inner data.
-        unsafe { mem::transmute::<&_, &mut _>(self.deref()) }
+        let inner = unsafe { &mut *self._ptr };
+        &mut inner.data
     }
 }