diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index da9e5fde7cc..43d60a4ed76 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -1849,6 +1849,8 @@ fn extend<'a, T, B: Extend<T>>(
     ///
     /// The relative order of partitioned items is not maintained.
     ///
+    /// Time Complexity: *O*(*N*)
+    ///
     /// See also [`is_partitioned()`] and [`partition()`].
     ///
     /// [`is_partitioned()`]: Iterator::is_partitioned