Add Router::has_routes (#2790)

axum/CHANGELOG.md

@@ -9,8 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - **change:** Avoid cloning `Arc` during deserialization of `Path`
 - **added:** `axum::serve::Serve::tcp_nodelay` and `axum::serve::WithGracefulShutdown::tcp_nodelay` ([#2653])
+- **added:** `Router::has_routes` function ([#2790])
 
 [#2653]: https://github.com/tokio-rs/axum/pull/2653
+[#2790]: https://github.com/tokio-rs/axum/pull/2790
 
 # 0.7.5 (24. March, 2024)
 

axum/src/docs/routing/route_layer.md

@@ -11,6 +11,10 @@ the request matches a route. This is useful for middleware that return early
 (such as authorization) which might otherwise convert a `404 Not Found` into a
 `401 Unauthorized`.
 
+This function will panic if no routes have been declared yet on the router,
+since the new layer will have no effect, and this is typically a bug.
+In generic code, you can test if that is the case first, by calling [`Router::has_routes`].
+
 # Example
 
 ```rust

axum/src/routing/mod.rs

@@ -307,6 +307,11 @@ where
         })
     }
 
+    /// True if the router currently has at least one route added.
+    pub fn has_routes(&self) -> bool {
+        self.inner.path_router.has_routes()
+    }
+
     #[track_caller]
     #[doc = include_str!("../docs/routing/fallback.md")]
     pub fn fallback<H, T>(self, handler: H) -> Self

axum/src/routing/path_router.rs

@@ -290,6 +290,10 @@ where
         }
     }
 
+    pub(super) fn has_routes(&self) -> bool {
+        !self.routes.is_empty()
+    }
+
     pub(super) fn with_state<S2>(self, state: S) -> PathRouter<S2, IS_FALLBACK> {
         let routes = self
             .routes