fix tests

Author: Qelxiros

library/alloc/src/vec/mod.rs

@@ -747,6 +747,7 @@ impl<T> Vec<T> {
     /// Basic usage:
     ///
     /// ```
+    /// #![feature(vec_peek_mut)]
     /// let mut vec = Vec::new();
     /// assert!(vec.peek_mut().is_none());
     ///

library/alloctests/lib.rs

@@ -42,7 +42,6 @@
 #![feature(trusted_random_access)]
 #![feature(try_reserve_kind)]
 #![feature(try_trait_v2)]
-#![feature(vec_peek_mut)]
 // tidy-alphabetical-end
 //
 // Language features:

library/alloctests/tests/lib.rs

@@ -40,6 +40,7 @@
 #![feature(vec_deque_truncate_front)]
 #![feature(unique_rc_arc)]
 #![feature(macro_metavar_expr_concat)]
+#![feature(vec_peek_mut)]
 #![allow(internal_features)]
 #![deny(fuzzy_provenance_casts)]
 #![deny(unsafe_op_in_unsafe_fn)]

library/alloctests/tests/vec.rs

@@ -2704,9 +2704,13 @@ fn test_peek_mut() {
     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));
+    if let Some(mut p) = vec.peek_mut() {
+        assert_eq!(*p, 2);
+        *p = 0;
+        assert_eq!(*p, 0);
+    } else {
+        unreachable!()
+    }
 }
 
 /// This assortment of tests, in combination with miri, verifies we handle UB on fishy arguments