std::process (unix): Implement From<io::Stdout> etc. for imp::Stdio

This involves adding a new variant `imp::Stdio::StaticFd`.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/sys/unix/process/process_common.rs

@@ -13,7 +13,7 @@
 use crate::sys::fs::File;
 use crate::sys::pipe::{self, AnonPipe};
 use crate::sys_common::process::{CommandEnv, CommandEnvs};
-use crate::sys_common::IntoInner;
+use crate::sys_common::{FromInner, IntoInner};
 
 #[cfg(not(target_os = "fuchsia"))]
 use crate::sys::fs::OpenOptions;
@@ -150,6 +150,7 @@ pub enum Stdio {
     Null,
     MakePipe,
     Fd(FileDesc),
+    StaticFd(BorrowedFd<'static>),
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
@@ -463,6 +464,11 @@ pub fn to_child_stdio(&self, readable: bool) -> io::Result<(ChildStdio, Option<A
                 }
             }
 
+            Stdio::StaticFd(fd) => {
+                let fd = FileDesc::from_inner(fd.try_clone_to_owned()?);
+                Ok((ChildStdio::Owned(fd), None))
+            }
+
             Stdio::MakePipe => {
                 let (reader, writer) = pipe::anon_pipe()?;
                 let (ours, theirs) = if readable { (writer, reader) } else { (reader, writer) };
@@ -497,6 +503,28 @@ fn from(file: File) -> Stdio {
     }
 }
 
+impl From<io::Stdout> for Stdio {
+    fn from(_: io::Stdout) -> Stdio {
+        // This ought really to be is Stdio::StaticFd(input_argument.as_fd()).
+        // But AsFd::as_fd takes its argument by reference, and yields
+        // a bounded lifetime, so it's no use here. There is no AsStaticFd.
+        //
+        // Additionally AsFd is only implemented for the *locked* versions.
+        // We don't want to lock them here.  (The implications of not locking
+        // are the same as those for process::Stdio::inherit().)
+        //
+        // Arguably the hypothetical AsStaticFd and AsFd<'static>
+        // should be implemented for io::Stdout, not just for StdoutLocked.
+        Stdio::StaticFd(unsafe { BorrowedFd::borrow_raw(libc::STDOUT_FILENO) })
+    }
+}
+
+impl From<io::Stderr> for Stdio {
+    fn from(_: io::Stderr) -> Stdio {
+        Stdio::StaticFd(unsafe { BorrowedFd::borrow_raw(libc::STDERR_FILENO) })
+    }
+}
+
 impl ChildStdio {
     pub fn fd(&self) -> Option<c_int> {
         match *self {