Rollup merge of rust-lang#71548 - crlf0710:cursor_bounds, r=Amanieu

Add missing Send and Sync impls for linked list Cursor and CursorMut.

Someone pointed out these to me, and i think it's indeed reasonable to add those impl.

r? @Amanieu

Author: Dylan-DPC

src/liballoc/collections/linked_list.rs

@@ -1841,3 +1841,15 @@ unsafe impl<T: Send> Send for IterMut<'_, T> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Sync> Sync for IterMut<'_, T> {}
+
+#[unstable(feature = "linked_list_cursors", issue = "58533")]
+unsafe impl<T: Sync> Send for Cursor<'_, T> {}
+
+#[unstable(feature = "linked_list_cursors", issue = "58533")]
+unsafe impl<T: Sync> Sync for Cursor<'_, T> {}
+
+#[unstable(feature = "linked_list_cursors", issue = "58533")]
+unsafe impl<T: Send> Send for CursorMut<'_, T> {}
+
+#[unstable(feature = "linked_list_cursors", issue = "58533")]
+unsafe impl<T: Sync> Sync for CursorMut<'_, T> {}