Require Output = () on WebSocketStream::on_upgrade (#646)

Fixes https://github.com/tokio-rs/axum/issues/636

axum/CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
-- None.
+- **breaking:** Require `Output = ()` on `WebSocketStream::on_upgrade` ([#644])
+
+[#644]: https://github.com/tokio-rs/axum/pull/644
 
 # 0.4.4 (13. January, 2021)
 

axum/src/extract/ws.rs

@@ -201,7 +201,7 @@ impl WebSocketUpgrade {
     pub fn on_upgrade<F, Fut>(self, callback: F) -> Response
     where
         F: FnOnce(WebSocket) -> Fut + Send + 'static,
-        Fut: Future + Send + 'static,
+        Fut: Future<Output = ()> + Send + 'static,
     {
         let on_upgrade = self.on_upgrade;
         let config = self.config;