Don't inherit fallbacks when using Router::nest_service

I think it makes sense to only inherit fallbacks for routers nested with
`Router::nest` and not `Router::nest_service`. It also simplifies the
code and avoids some unfortunate extension work arounds.

axum/CHANGELOG.md

@@ -50,6 +50,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   type inference issues when calling `ServiceExt` methods on a `Router` ([#1835])
 - **breaking:** Removed `axum::Server` as it was removed in hyper 1.0. Instead
   use `axum::serve(listener, service)` or hyper/hyper-util for more configuration options ([#1868])
+- **breaking:** Only inherit fallbacks for routers nested with `Router::nest`.
+  Routers nested with `Router::nest_service` will no longer inherit fallbacks ([#1956])
 
 [#1664]: https://github.com/tokio-rs/axum/pull/1664
 [#1751]: https://github.com/tokio-rs/axum/pull/1751
@@ -58,6 +60,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [#1835]: https://github.com/tokio-rs/axum/pull/1835
 [#1850]: https://github.com/tokio-rs/axum/pull/1850
 [#1868]: https://github.com/tokio-rs/axum/pull/1868
+[#1956]: https://github.com/tokio-rs/axum/pull/1956
 
 # 0.6.16 (18. April, 2023)
 

axum/src/routing/mod.rs

@@ -19,7 +19,6 @@ use std::{
     marker::PhantomData,
     task::{Context, Poll},
 };
-use sync_wrapper::SyncWrapper;
 use tower_layer::Layer;
 use tower_service::Service;
 
@@ -285,43 +284,15 @@ where
         }
     }
 
-    pub(crate) fn call_with_state(
-        &mut self,
-        mut req: Request,
-        state: S,
-    ) -> RouteFuture<Infallible> {
-        // required for opaque routers to still inherit the fallback
-        // TODO(david): remove this feature in 0.7
-        if !self.default_fallback {
-            req.extensions_mut().insert(SuperFallback(SyncWrapper::new(
-                self.fallback_router.clone(),
-            )));
-        }
-
+    pub(crate) fn call_with_state(&mut self, req: Request, state: S) -> RouteFuture<Infallible> {
         match self.path_router.call_with_state(req, state) {
             Ok(future) => future,
-            Err((mut req, state)) => {
-                let super_fallback = req
-                    .extensions_mut()
-                    .remove::<SuperFallback<S>>()
-                    .map(|SuperFallback(path_router)| path_router.into_inner());
-
-                if let Some(mut super_fallback) = super_fallback {
-                    return super_fallback
-                        .call_with_state(req, state)
-                        .unwrap_or_else(|_| unreachable!());
-                }
-
-                match self.fallback_router.call_with_state(req, state) {
-                    Ok(future) => future,
-                    Err((_req, _state)) => {
-                        unreachable!(
-                            "the default fallback added in `Router::new` \
-                             matches everything"
-                        )
-                    }
+            Err((req, state)) => match self.fallback_router.call_with_state(req, state) {
+                Ok(future) => future,
+                Err((_req, _state)) => {
+                    unreachable!("the default fallback added in `Router::new` matches everything")
                 }
-            }
+            },
         }
     }
 
@@ -662,8 +633,6 @@ impl<S> fmt::Debug for Endpoint<S> {
     }
 }
 
-struct SuperFallback<S>(SyncWrapper<PathRouter<S, true>>);
-
 #[test]
 #[allow(warnings)]
 fn traits() {

axum/src/routing/tests/fallback.rs

@@ -1,5 +1,3 @@
-use tower::ServiceExt;
-
 use super::*;
 use crate::middleware::{map_request, map_response};
 
@@ -166,48 +164,6 @@ async fn also_inherits_default_layered_fallback() {
     assert_eq!(res.text().await, "outer");
 }
 
-#[crate::test]
-async fn fallback_inherited_into_nested_router_service() {
-    let inner = Router::new()
-        .route(
-            "/bar",
-            get(|State(state): State<&'static str>| async move { state }),
-        )
-        .with_state("inner");
-
-    // with a different state
-    let app = Router::new()
-        .nest_service("/foo", inner)
-        .fallback(outer_fallback);
-
-    let client = TestClient::new(app);
-    let res = client.get("/foo/not-found").send().await;
-    assert_eq!(res.status(), StatusCode::NOT_FOUND);
-    assert_eq!(res.text().await, "outer");
-}
-
-#[crate::test]
-async fn fallback_inherited_into_nested_opaque_service() {
-    let inner = Router::new()
-        .route(
-            "/bar",
-            get(|State(state): State<&'static str>| async move { state }),
-        )
-        .with_state("inner")
-        // even if the service is made more opaque it should still inherit the fallback
-        .boxed_clone();
-
-    // with a different state
-    let app = Router::new()
-        .nest_service("/foo", inner)
-        .fallback(outer_fallback);
-
-    let client = TestClient::new(app);
-    let res = client.get("/foo/not-found").send().await;
-    assert_eq!(res.status(), StatusCode::NOT_FOUND);
-    assert_eq!(res.text().await, "outer");
-}
-
 #[crate::test]
 async fn nest_fallback_on_inner() {
     let app = Router::new()