Add method_not_allowed_fallback to router (#2903)

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>

axum/src/docs/routing/method_not_allowed_fallback.md

@@ -0,0 +1,38 @@
+Add a fallback [`Handler`] for the case where a route exists, but the method of the request is not supported.
+
+Sets a fallback on all previously registered [`MethodRouter`]s,
+to be called when no matching method handler is set.
+
+```rust,no_run
+use axum::{response::IntoResponse, routing::get, Router};
+
+async fn hello_world() -> impl IntoResponse {
+    "Hello, world!\n"
+}
+
+async fn default_fallback() -> impl IntoResponse {
+    "Default fallback\n"
+}
+
+async fn handle_405() -> impl IntoResponse {
+    "Method not allowed fallback"
+}
+
+#[tokio::main]
+async fn main() {
+    let router = Router::new()
+        .route("/", get(hello_world))
+        .fallback(default_fallback)
+        .method_not_allowed_fallback(handle_405);
+
+    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
+
+    axum::serve(listener, router).await.unwrap();
+}
+```
+
+The fallback only applies if there is a `MethodRouter` registered for a given path, 
+but the method used in the request is not specified. In the example, a `GET` on 
+`http://localhost:3000` causes the `hello_world` handler to react, while issuing a 
+`POST` triggers `handle_405`. Calling an entirely different route, like `http://localhost:3000/hello` 
+causes `default_fallback` to run.

axum/src/routing/method_routing.rs

@@ -659,6 +659,19 @@ where
         self.fallback = Fallback::BoxedHandler(BoxedIntoRoute::from_handler(handler));
         self
     }
+
+    /// Add a fallback [`Handler`] if no custom one has been provided.
+    pub(crate) fn default_fallback<H, T>(self, handler: H) -> Self
+    where
+        H: Handler<T, S>,
+        T: 'static,
+        S: Send + Sync + 'static,
+    {
+        match self.fallback {
+            Fallback::Default(_) => self.fallback(handler),
+            _ => self,
+        }
+    }
 }
 
 impl MethodRouter<(), Infallible> {

axum/src/routing/mod.rs

@@ -354,6 +354,18 @@ where
         .fallback_endpoint(Endpoint::Route(route))
     }
 
+    #[doc = include_str!("../docs/routing/method_not_allowed_fallback.md")]
+    pub fn method_not_allowed_fallback<H, T>(self, handler: H) -> Self
+    where
+        H: Handler<T, S>,
+        T: 'static,
+    {
+        tap_inner!(self, mut this => {
+            this.path_router
+                .method_not_allowed_fallback(handler.clone())
+        })
+    }
+
     fn fallback_endpoint(self, endpoint: Endpoint<S>) -> Self {
         tap_inner!(self, mut this => {
             this.fallback_router.set_fallback(endpoint);

axum/src/routing/path_router.rs

@@ -1,4 +1,7 @@
-use crate::extract::{nested_path::SetNestedPath, Request};
+use crate::{
+    extract::{nested_path::SetNestedPath, Request},
+    handler::Handler,
+};
 use axum_core::response::IntoResponse;
 use matchit::MatchError;
 use std::{borrow::Cow, collections::HashMap, convert::Infallible, fmt, sync::Arc};
@@ -110,6 +113,18 @@ where
         Ok(())
     }
 
+    pub(super) fn method_not_allowed_fallback<H, T>(&mut self, handler: H)
+    where
+        H: Handler<T, S>,
+        T: 'static,
+    {
+        for (_, endpoint) in self.routes.iter_mut() {
+            if let Endpoint::MethodRouter(rt) = endpoint {
+                *rt = rt.clone().default_fallback(handler.clone());
+            }
+        }
+    }
+
     pub(super) fn route_service<T>(
         &mut self,
         path: &str,

axum/src/routing/tests/fallback.rs

@@ -329,6 +329,51 @@ async fn merge_router_with_fallback_into_empty() {
     assert_eq!(res.text().await, "outer");
 }
 
+#[crate::test]
+async fn mna_fallback_with_existing_fallback() {
+    let app = Router::new()
+        .route(
+            "/",
+            get(|| async { "test" }).fallback(|| async { "index fallback" }),
+        )
+        .route("/path", get(|| async { "path" }))
+        .method_not_allowed_fallback(|| async { "method not allowed fallback" });
+
+    let client = TestClient::new(app);
+    let index_fallback = client.post("/").await;
+    let method_not_allowed_fallback = client.post("/path").await;
+
+    assert_eq!(index_fallback.text().await, "index fallback");
+    assert_eq!(
+        method_not_allowed_fallback.text().await,
+        "method not allowed fallback"
+    );
+}
+
+#[crate::test]
+async fn mna_fallback_with_state() {
+    let app = Router::new()
+        .route("/", get(|| async { "index" }))
+        .method_not_allowed_fallback(|State(state): State<&'static str>| async move { state })
+        .with_state("state");
+
+    let client = TestClient::new(app);
+    let res = client.post("/").await;
+    assert_eq!(res.text().await, "state");
+}
+
+#[crate::test]
+async fn mna_fallback_with_unused_state() {
+    let app = Router::new()
+        .route("/", get(|| async { "index" }))
+        .with_state(())
+        .method_not_allowed_fallback(|| async move { "bla" });
+
+    let client = TestClient::new(app);
+    let res = client.post("/").await;
+    assert_eq!(res.text().await, "bla");
+}
+
 #[crate::test]
 async fn state_isnt_cloned_too_much_with_fallback() {
     let state = CountingCloneableState::new();