process: allow for ergonomically piping stdio of one child into another (#3466)

* Add `TryInto<Stdio>` impls for `ChildStd{in,out,err}` for ergonomic
  conversions into `std::process::Stdio` so callers can perform the
  conversion without needing to manipulate raw fds/handles directly

tests-integration/tests/process_stdio.rs

@@ -1,11 +1,13 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
-use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+use tokio::io::{AsyncBufReadExt, AsyncReadExt, AsyncWriteExt, BufReader};
+use tokio::join;
 use tokio::process::{Child, Command};
 use tokio_test::assert_ok;
 
 use futures::future::{self, FutureExt};
+use std::convert::TryInto;
 use std::env;
 use std::io;
 use std::process::{ExitStatus, Stdio};
@@ -139,3 +141,52 @@ async fn try_wait() {
     // Can't get id after process has exited
     assert_eq!(child.id(), None);
 }
+
+#[tokio::test]
+async fn pipe_from_one_command_to_another() {
+    let mut first = cat().spawn().expect("first cmd");
+    let mut third = cat().spawn().expect("third cmd");
+
+    // Convert ChildStdout to Stdio
+    let second_stdin: Stdio = first
+        .stdout
+        .take()
+        .expect("first.stdout")
+        .try_into()
+        .expect("first.stdout into Stdio");
+
+    // Convert ChildStdin to Stdio
+    let second_stdout: Stdio = third
+        .stdin
+        .take()
+        .expect("third.stdin")
+        .try_into()
+        .expect("third.stdin into Stdio");
+
+    let mut second = cat()
+        .stdin(second_stdin)
+        .stdout(second_stdout)
+        .spawn()
+        .expect("first cmd");
+
+    let msg = "hello world! please pipe this message through";
+
+    let mut stdin = first.stdin.take().expect("first.stdin");
+    let write = async move { stdin.write_all(msg.as_bytes()).await };
+
+    let mut stdout = third.stdout.take().expect("third.stdout");
+    let read = async move {
+        let mut data = String::new();
+        stdout.read_to_string(&mut data).await.map(|_| data)
+    };
+
+    let (read, write, first_status, second_status, third_status) =
+        join!(read, write, first.wait(), second.wait(), third.wait());
+
+    assert_eq!(msg, read.expect("read result"));
+    write.expect("write result");
+
+    assert!(first_status.expect("first status").success());
+    assert!(second_status.expect("second status").success());
+    assert!(third_status.expect("third status").success());
+}