test: implement Drop for Mock to panic w/ unconsumed data (#2704)

tokio-test/src/io.rs

@@ -458,6 +458,21 @@ impl AsyncWrite for Mock {
     }
 }
 
+/// Ensures that Mock isn't dropped with data "inside".
+impl Drop for Mock {
+    fn drop(&mut self) {
+        // Avoid double panicking, since makes debugging much harder.
+        if std::thread::panicking() {
+            return;
+        }
+
+        self.inner.actions.iter().for_each(|a| match a {
+            Action::Read(data) => assert!(data.is_empty(), "There is still data left to read."),
+            Action::Write(data) => assert!(data.is_empty(), "There is still data left to write."),
+            _ => (),
+        })
+    }
+}
 /*
 /// Returns `true` if called from the context of a futures-rs Task
 fn is_task_ctx() -> bool {

tokio-test/tests/io.rs

@@ -70,3 +70,17 @@ async fn write_error() {
 
     mock.write_all(b"world!").await.expect("write 2");
 }
+
+#[tokio::test]
+#[should_panic]
+async fn mock_panics_read_data_left() {
+    use tokio_test::io::Builder;
+    Builder::new().read(b"read").build();
+}
+
+#[tokio::test]
+#[should_panic]
+async fn mock_panics_write_data_left() {
+    use tokio_test::io::Builder;
+    Builder::new().write(b"write").build();
+}