Add a small example for listening to both ipv4 and ipv6

Presenting stream merge on Incoming.

Author: skade

examples/tcp-ipv4-and-6-echo.rs

@@ -0,0 +1,44 @@
+//! TCP echo server, accepting connections both on both ipv4 and ipv6 sockets.
+//!
+//! To send messages, do:
+//!
+//! ```sh
+//! $ nc 127.0.0.1 8080
+//! $ nc ::1 8080
+//! ```
+
+use async_std::io;
+use async_std::net::{TcpListener, TcpStream};
+use async_std::prelude::*;
+use async_std::task;
+
+async fn process(stream: TcpStream) -> io::Result<()> {
+    println!("Accepted from: {}", stream.peer_addr()?);
+
+    let (reader, writer) = &mut (&stream, &stream);
+    io::copy(reader, writer).await?;
+
+    Ok(())
+}
+
+fn main() -> io::Result<()> {
+    task::block_on(async {
+        let ipv4_listener = TcpListener::bind("127.0.0.1:8080").await?;
+        println!("Listening on {}", ipv4_listener.local_addr()?);
+        let ipv6_listener = TcpListener::bind("[::1]:8080").await?;
+        println!("Listening on {}", ipv6_listener.local_addr()?);
+
+        let ipv4_incoming = ipv4_listener.incoming();
+        let ipv6_incoming = ipv6_listener.incoming();
+
+        let mut incoming = ipv4_incoming.merge(ipv6_incoming);
+
+        while let Some(stream) = incoming.next().await {
+            let stream = stream?;
+            task::spawn(async {
+                process(stream).await.unwrap();
+            });
+        }
+        Ok(())
+    })
+}