From 2138489ce55d6d1ec98d3df7c332a753383a5bb4 Mon Sep 17 00:00:00 2001
From: David Pedersen <david.pdrsn@gmail.com>
Date: Wed, 2 Aug 2023 21:17:12 +0200
Subject: [PATCH] Implement `Handler` for `T: IntoResponse` (#2140)

---
 axum/CHANGELOG.md             |  2 ++
 axum/src/handler/mod.rs       | 42 +++++++++++++++++++++++++++++++++++
 axum/src/routing/tests/mod.rs | 11 +++++++++
 3 files changed, 55 insertions(+)

diff --git a/axum/CHANGELOG.md b/axum/CHANGELOG.md
index 5e1d181ecf..8cbca27937 100644
--- a/axum/CHANGELOG.md
+++ b/axum/CHANGELOG.md
@@ -62,6 +62,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **fixed:** Allow unreachable code in `#[debug_handler]` ([#2014])
 - **change:** Update tokio-tungstenite to 0.19 ([#2021])
 - **change:** axum's MSRV is now 1.63 ([#2021])
+- **added:** Implement `Handler` for `T: IntoResponse` ([#2140])
 
 [#2021]: https://github.com/tokio-rs/axum/pull/2021
 [#2014]: https://github.com/tokio-rs/axum/pull/2014
@@ -78,6 +79,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [#2058]: https://github.com/tokio-rs/axum/pull/2058
 [#2073]: https://github.com/tokio-rs/axum/pull/2073
 [#2096]: https://github.com/tokio-rs/axum/pull/2096
+[#2140]: https://github.com/tokio-rs/axum/pull/2140
 
 # 0.6.17 (25. April, 2023)
 
diff --git a/axum/src/handler/mod.rs b/axum/src/handler/mod.rs
index 69ff8c8025..ea352b9a2a 100644
--- a/axum/src/handler/mod.rs
+++ b/axum/src/handler/mod.rs
@@ -100,6 +100,31 @@ pub use self::service::HandlerService;
 /// {}
 /// ```
 #[doc = include_str!("../docs/debugging_handler_type_errors.md")]
+///
+/// # Handlers that aren't functions
+///
+/// The `Handler` trait is also implemented for `T: IntoResponse`. That allows easily returning
+/// fixed data for routes:
+///
+/// ```
+/// use axum::{
+///     Router,
+///     routing::{get, post},
+///     Json,
+///     http::StatusCode,
+/// };
+/// use serde_json::json;
+///
+/// let app = Router::new()
+///     // respond with a fixed string
+///     .route("/", get("Hello, World!"))
+///     // or return some mock data
+///     .route("/users", post((
+///         StatusCode::CREATED,
+///         Json(json!({ "id": 1, "username": "alice" })),
+///     )));
+/// # let _: Router = app;
+/// ```
 #[cfg_attr(
     nightly_error_messages,
     rustc_on_unimplemented(
@@ -224,6 +249,23 @@ macro_rules! impl_handler {
 
 all_the_tuples!(impl_handler);
 
+mod private {
+    // Marker type for `impl<T: IntoResponse> Handler for T`
+    #[allow(missing_debug_implementations)]
+    pub enum IntoResponseHandler {}
+}
+
+impl<T, S> Handler<private::IntoResponseHandler, S> for T
+where
+    T: IntoResponse + Clone + Send + 'static,
+{
+    type Future = std::future::Ready<Response>;
+
+    fn call(self, _req: Request, _state: S) -> Self::Future {
+        std::future::ready(self.into_response())
+    }
+}
+
 /// A [`Service`] created from a [`Handler`] by applying a Tower middleware.
 ///
 /// Created with [`Handler::layer`]. See that method for more details.
diff --git a/axum/src/routing/tests/mod.rs b/axum/src/routing/tests/mod.rs
index 027d735eb0..2bfc84526b 100644
--- a/axum/src/routing/tests/mod.rs
+++ b/axum/src/routing/tests/mod.rs
@@ -1021,3 +1021,14 @@ async fn connect_going_to_default_fallback() {
     let body = hyper::body::to_bytes(res).await.unwrap();
     assert!(body.is_empty());
 }
+
+#[crate::test]
+async fn impl_handler_for_into_response() {
+    let app = Router::new().route("/things", post((StatusCode::CREATED, "thing created")));
+
+    let client = TestClient::new(app);
+
+    let res = client.post("/things").send().await;
+    assert_eq!(res.status(), StatusCode::CREATED);
+    assert_eq!(res.text().await, "thing created");
+}