rust-chg: have attach_io() simply take reference of AsRawFd object

We no longer have to deal with the restriction of the Future type.
Before, these file objects couldn't be references and that's the only
reason why we had to make stderr an Option<T>.

This fixes future type deduction issue of stderr = None, where rustc would
complain that T of Option<T> couldn't be deduced.

Differential Revision: https://phab.mercurial-scm.org/D8443

rust/chg/src/attachio.rs

@@ -21,14 +21,12 b' use crate::procutil;'
 /// 3. Client sends fds with 1-byte dummy payload in response.
 /// 4. Server returns the number of the fds received.
 ///
-/// If the stderr is omitted, it will be redirected to the stdout. This
-/// allows us to attach the pager stdin to both stdout and stderr, and
-/// dispose of the client-side handle once attached.
+/// The client-side fds may be dropped once duplicated to the server.
 pub async fn attach_io(
     proto: &mut Protocol<impl Connection + AsRawFd>,
-    stdin: impl AsRawFd,
-    stdout: impl AsRawFd,
-    stderr: Option<impl AsRawFd>,
+    stdin: &impl AsRawFd,
+    stdout: &impl AsRawFd,
+    stderr: &impl AsRawFd,
 ) -> io::Result<()> {
     // TODO: unindent
     {
@@ -56,7 +54,7 b' pub async fn attach_io('
                     let sock_fd = proto.as_raw_fd();
                     let ifd = stdin.as_raw_fd();
                     let ofd = stdout.as_raw_fd();
-                    let efd = stderr.as_ref().map_or(ofd, |f| f.as_raw_fd());
+                    let efd = stderr.as_raw_fd();
                     procutil::send_raw_fds(sock_fd, &[ifd, ofd, efd])?;
                 }
                 ChannelMessage::InputRequest(..)