task: add JoinMap::keys (#6046)

Co-authored-by: Alice Ryhl <[email protected]>

tokio-util/tests/task_join_map.rs

@@ -109,6 +109,30 @@ async fn alternating() {
     }
 }
 
+#[tokio::test]
+async fn test_keys() {
+    use std::collections::HashSet;
+
+    let mut map = JoinMap::new();
+
+    assert_eq!(map.len(), 0);
+    map.spawn(1, async {});
+    assert_eq!(map.len(), 1);
+    map.spawn(2, async {});
+    assert_eq!(map.len(), 2);
+
+    let keys = map.keys().collect::<HashSet<&u32>>();
+    assert!(keys.contains(&1));
+    assert!(keys.contains(&2));
+
+    let _ = map.join_next().await.unwrap();
+    let _ = map.join_next().await.unwrap();
+
+    assert_eq!(map.len(), 0);
+    let keys = map.keys().collect::<HashSet<&u32>>();
+    assert!(keys.is_empty());
+}
+
 #[tokio::test(start_paused = true)]
 async fn abort_by_key() {
     let mut map = JoinMap::new();