add Vec::peek_mut

2 files changed, 12 insertions, 0 deletions

diff --git a/library/alloctests/lib.rs b/library/alloctests/lib.rs
index 56e60ed4c84..232cf06fff9 100644
--- a/library/alloctests/lib.rs
+++ b/library/alloctests/lib.rs
@@ -42,6 +42,7 @@
 #![feature(trusted_random_access)]
 #![feature(try_reserve_kind)]
 #![feature(try_trait_v2)]
+#![feature(vec_peek_mut)]
 // tidy-alphabetical-end
 //
 // Language features:
diff --git a/library/alloctests/tests/vec.rs b/library/alloctests/tests/vec.rs
index f430d979fa8..0de9da37561 100644
--- a/library/alloctests/tests/vec.rs
+++ b/library/alloctests/tests/vec.rs
@@ -2698,6 +2698,17 @@ fn test_pop_if_mutates() {
     assert_eq!(v, [2]);
 }
 
+#[test]
+fn test_peek_mut() {
+    let mut vec = Vec::new();
+    assert!(vec.peek_mut().is_none());
+    vec.push(1);
+    vec.push(2);
+    assert_eq!(vec.peek_mut(), Some(2));
+    *vec.peek_mut() = 0;
+    assert_eq!(vec.peek_mut(), Some(0));
+}
+
 /// This assortment of tests, in combination with miri, verifies we handle UB on fishy arguments
 /// in the stdlib. Draining and extending the allocation are fairly well-tested earlier, but
 /// `vec.insert(usize::MAX, val)` once slipped by!