diff --git a/src/trait/impl_trait.md b/src/trait/impl_trait.md
index 7b87bd2e9c..f429d9adf7 100644
--- a/src/trait/impl_trait.md
+++ b/src/trait/impl_trait.md
@@ -68,4 +68,10 @@ fn double_positives<'a>(numbers: &'a Vec<i32>) -> impl Iterator<Item = i32> + 'a
         .filter(|x| x > &&0)
         .map(|x| x * 2)
 }
+
+fn main() {
+    let singles = vec![-3, -2, 2, 3];
+    let doubles = double_positives(&singles);
+    assert_eq!(doubles.collect::<Vec<i32>>(), vec![4, 6]);
+}
 ```