diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index 18e191f2b59..f985fb78465 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -125,7 +125,7 @@ fn as_ref(&self) -> &[T] {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Send, A: Allocator + Send> Send for IntoIter<T, A> {}
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<T: Sync, A: Allocator> Sync for IntoIter<T, A> {}
+unsafe impl<T: Sync, A: Allocator + Sync> Sync for IntoIter<T, A> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator> Iterator for IntoIter<T, A> {