Add a method to test if split streams come from the same stream. (#1762)

* Add a method to test if split streams come from the same stream.

The exposed stream ID can also be used as key in associative containers.

* Document the fact that split stream IDs can dangle.

tokio/tests/io_split.rs

@@ -45,6 +45,16 @@ fn is_send_and_sync() {
     assert_bound::<WriteHalf<RW>>();
 }
 
+#[test]
+fn split_stream_id() {
+    let (r1, w1) = split(RW);
+    let (r2, w2) = split(RW);
+    assert_eq!(r1.stream_id(), w1.stream_id());
+    assert_eq!(r1.stream_id(), w1.stream_id());
+    assert_ne!(r1.stream_id(), w2.stream_id());
+    assert_ne!(r2.stream_id(), w1.stream_id());
+}
+
 #[test]
 fn unsplit_ok() {
     let (r, w) = split(RW);