Fix doc test for Vec::retain(), now passes clippy::eval_order_dependence

library/alloc/src/vec/mod.rs

@@ -1385,13 +1385,14 @@ impl<T, A: Allocator> Vec<T, A> {
     /// assert_eq!(vec, [2, 4]);
     /// ```
     ///
-    /// The exact order may be useful for tracking external state, like an index.
+    /// Because the elements are visited exactly once in the original order,
+    /// external state may be used to decide which elements to keep.
     ///
     /// ```
     /// let mut vec = vec![1, 2, 3, 4, 5];
     /// let keep = [false, true, true, false, true];
-    /// let mut i = 0;
-    /// vec.retain(|_| (keep[i], i += 1).0);
+    /// let mut iter = keep.iter();
+    /// vec.retain(|_| *iter.next().unwrap());
     /// assert_eq!(vec, [2, 3, 5]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]