tokio-fs: rewrite std echo example using async/await (#1442)

This PR fixes the echo example in tokio-fs.

Refs: #1255

tokio-fs/examples/std-echo.rs

@@ -0,0 +1,34 @@
+//! Echo everything received on STDIN to STDOUT and STDERR.
+#![feature(async_await)]
+
+use futures_util::{FutureExt, SinkExt, StreamExt, TryFutureExt};
+
+use tokio::codec::{FramedRead, FramedWrite, LinesCodec, LinesCodecError};
+use tokio::future::ready;
+use tokio_fs::{stderr, stdin, stdout};
+use tokio_threadpool::Builder;
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let pool = Builder::new().pool_size(1).build();
+
+    pool.spawn(
+        async {
+            let mut input = FramedRead::new(stdin(), LinesCodec::new());
+            let mut output = FramedWrite::new(stdout(), LinesCodec::new());
+            let mut error = FramedWrite::new(stderr(), LinesCodec::new());
+
+            while let Some(line) = input.next().await {
+                let line = line?;
+                output.send(format!("OUT: {}", line)).await?;
+                error.send(format!("ERR: {}", line)).await?;
+            }
+            Ok(())
+        }
+            .map_err(|e: LinesCodecError| panic!(e))
+            .then(|_| ready(())),
+    );
+
+    pool.shutdown_on_idle().await;
+    Ok(())
+}

tokio-fs/examples_old/std-echo.rs

@@ -1,43 +0,0 @@
-//! Echo everything received on STDIN to STDOUT.
-#![feature(async_await)]
-
-use tokio_codec::{FramedRead, FramedWrite, LinesCodec};
-use tokio_fs::{stderr, stdin, stdout};
-use tokio_threadpool::Builder;
-
-use futures_util::sink::SinkExt;
-
-use std::io;
-
-#[tokio::main]
-async fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let pool = Builder::new().pool_size(1).build();
-
-    pool.spawn({
-        let input = FramedRead::new(stdin(), LinesCodec::new());
-
-        let output = FramedWrite::new(stdout(), LinesCodec::new()).with(|line: String| {
-            let mut out = "OUT: ".to_string();
-            out.push_str(&line);
-            Ok::<_, io::Error>(out)
-        });
-
-        let error = FramedWrite::new(stderr(), LinesCodec::new()).with(|line: String| {
-            let mut out = "ERR: ".to_string();
-            out.push_str(&line);
-            Ok::<_, io::Error>(out)
-        });
-
-        let dst = output.fanout(error);
-
-        input
-            .forward(dst)
-            .map(|_| ())
-            .map_err(|e| panic!("io error = {:?}", e))
-    });
-
-    pool.shutdown_on_idle()
-        .wait()
-        .map_err(|_| "failed to shutdown the thread pool")?;
-    Ok(())
-}