fix nested fallback actionable also on nested root

Author: dalton-oliveira

Diff for: axum/CHANGELOG.md

@@ -7,7 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
-- None.
+- **fixed:** Fallback handlers on nested routers returning 404
 
 # 0.7.4 (13. January, 2024)
 

Diff for: axum/src/routing/path_router.rs

@@ -180,16 +180,27 @@ where
                 .expect("no path for route id. This is a bug in axum. Please file an issue");
 
             let path = path_for_nested_route(prefix, inner_path);
-
+            let route_nested_root =
+                IS_FALLBACK && "/".eq(inner_path.as_ref()) && !path.ends_with('/');
+            let nested_root = match route_nested_root {
+                true => Some(format!("{path}/")),
+                false => None,
+            };
             let layer = (
                 StripPrefix::layer(prefix),
                 SetNestedPath::layer(path_to_nest_at),
             );
             match endpoint.layer(layer) {
                 Endpoint::MethodRouter(method_router) => {
+                    if let Some(nested_root) = nested_root {
+                        self.route(&nested_root, method_router.clone())?;
+                    }
                     self.route(&path, method_router)?;
                 }
                 Endpoint::Route(route) => {
+                    if let Some(nested_root) = nested_root {
+                        self.route_endpoint(&nested_root, Endpoint::Route(route.clone()))?;
+                    }
                     self.route_endpoint(&path, Endpoint::Route(route))?;
                 }
             }

Diff for: axum/src/routing/tests/nest.rs

@@ -383,6 +383,19 @@ async fn nesting_with_root_inner_router() {
     assert_eq!(res.status(), StatusCode::OK);
 }
 
+#[tokio::test]
+async fn nesting_with_root_inner_fallback() {
+    let app = Router::new().nest("/router", Router::new().fallback(get(|| async {})));
+
+    let client = TestClient::new(app);
+
+    let res = client.get("/router").await;
+    assert_eq!(res.status(), StatusCode::OK);
+
+    let res = client.get("/router/").await;
+    assert_eq!(res.status(), StatusCode::OK);
+}
+
 macro_rules! nested_route_test {
     (
         $name:ident,