Add example showing how to use hyper's low level API

If `axum::serve` doesn't work for people we can point them to this
example.

axum/src/extract/connect_info.rs

@@ -94,6 +94,12 @@ const _: () = {
     }
 };
 
+impl Connected<SocketAddr> for SocketAddr {
+    fn connect_info(remote_addr: SocketAddr) -> Self {
+        remote_addr
+    }
+}
+
 impl<S, C, T> Service<T> for IntoMakeServiceWithConnectInfo<S, C>
 where
     S: Clone,

examples/serve-with-hyper/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "example-serve-with-hyper"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+tokio = { version = "1.0", features = ["full"] }
+hyper-util = { version = "0.1", features = ["tokio", "server-auto", "http1"] }
+hyper = { version = "1.0", features = [] }
+tower = { version = "0.4", features  = ["util"] }

examples/serve-with-hyper/src/main.rs

@@ -0,0 +1,124 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-serve-with-hyper
+//! ```
+//!
+//! This example shows how to run axum using hyper's low level API.
+//!
+//! The [hyper-util] crate exists to provide high level utilities but its still in early stages of
+//! development.
+//!
+//! [hyper-util]: https://crates.io/crates/hyper-util
+
+use std::convert::Infallible;
+use std::net::SocketAddr;
+
+use axum::extract::ConnectInfo;
+use axum::{extract::Request, routing::get, Router};
+use hyper::body::Incoming;
+use hyper_util::rt::{TokioExecutor, TokioIo};
+use hyper_util::server;
+use tokio::net::TcpListener;
+use tower::{Service, ServiceExt};
+
+#[tokio::main]
+async fn main() {
+    tokio::join!(serve_plain(), serve_with_connect_info());
+}
+
+async fn serve_plain() {
+    // Create a regular axum app.
+    let app = Router::new().route("/", get(|| async { "Hello!" }));
+
+    // Create a `TcpListener` using tokio.
+    let listener = TcpListener::bind("0.0.0.0:3000").await.unwrap();
+
+    // Continuously accept new connections.
+    loop {
+        // In this example we discard the remote address. See `fn serve_with_connect_info` for how
+        // to expose that.
+        let (socket, _remote_addr) = listener.accept().await.unwrap();
+
+        // We don't need to call `poll_ready` because `Router` is always ready.
+        let tower_service = app.clone();
+
+        // Spawn a task to handle the connection. That way we can multiple connections
+        // concurrently.
+        tokio::spawn(async move {
+            // Hyper has its own `AsyncRead` and `AsyncWrite` traits and doesn't use tokio.
+            // `TokioIo` converts between them.
+            let socket = TokioIo::new(socket);
+
+            // Hyper also has its own `Service` trait and doesn't use tower. We can use
+            // `hyper::service::service_fn` to create a hyper `Service` that calls our app through
+            // `tower::Service::call`.
+            let hyper_service = hyper::service::service_fn(move |request: Request<Incoming>| {
+                // We have to clone `tower_service` because hyper's `Service` uses `&self` whereas
+                // tower's `Service` requires `&mut self`.
+                //
+                // We don't need to call `poll_ready` since `Router` is always ready.
+                tower_service.clone().call(request)
+            });
+
+            // `server::conn::auto::Builder` supports both http1 and http2.
+            //
+            // `TokioExecutor` tells hyper to use `tokio::spawn` to spawn tasks.
+            if let Err(err) = server::conn::auto::Builder::new(TokioExecutor::new())
+                // `serve_connection_with_upgrades` is required for websockets. If you don't need
+                // that you can use `serve_connection` instead.
+                .serve_connection_with_upgrades(socket, hyper_service)
+                .await
+            {
+                eprintln!("failed to serve connection: {err:#}");
+            }
+        });
+    }
+}
+
+// Similar setup to `serve_plain` but captures the remote address and exposes it through the
+// `ConnectInfo` extractor
+async fn serve_with_connect_info() {
+    let app = Router::new().route(
+        "/",
+        get(
+            |ConnectInfo(remote_addr): ConnectInfo<SocketAddr>| async move {
+                format!("Hello {remote_addr}")
+            },
+        ),
+    );
+
+    let mut make_service = app.into_make_service_with_connect_info::<SocketAddr>();
+
+    let listener = TcpListener::bind("0.0.0.0:3001").await.unwrap();
+
+    loop {
+        let (socket, remote_addr) = listener.accept().await.unwrap();
+
+        // We don't need to call `poll_ready` because `IntoMakeServiceWithConnectInfo` is always
+        // ready.
+        let tower_service = unwrap_infallible(make_service.call(remote_addr).await);
+
+        tokio::spawn(async move {
+            let socket = TokioIo::new(socket);
+
+            let hyper_service = hyper::service::service_fn(move |request: Request<Incoming>| {
+                tower_service.clone().oneshot(request)
+            });
+
+            if let Err(err) = server::conn::auto::Builder::new(TokioExecutor::new())
+                .serve_connection_with_upgrades(socket, hyper_service)
+                .await
+            {
+                eprintln!("failed to serve connection: {err:#}");
+            }
+        });
+    }
+}
+
+fn unwrap_infallible<T>(result: Result<T, Infallible>) -> T {
+    match result {
+        Ok(value) => value,
+        Err(err) => match err {},
+    }
+}