Use Infallible in the exec_monitor return type

This makes it clearer that exec_monitor can only return when there is an
error.

src/exec/use_pty/monitor.rs

@@ -1,4 +1,4 @@
-use std::{ffi::c_int, io, os::unix::process::CommandExt, process::Command};
+use std::{convert::Infallible, ffi::c_int, io, os::unix::process::CommandExt, process::Command};
 
 use crate::exec::{opt_fmt, signal_fmt};
 use crate::system::signal::{
@@ -17,7 +17,6 @@ use crate::{
     exec::{
         event::{PollEvent, StopReason},
         use_pty::{SIGCONT_BG, SIGCONT_FG},
-        ExecOutput,
     },
     log::{dev_error, dev_info, dev_warn},
     system::FileCloser,
@@ -36,15 +35,14 @@ use crate::{
 
 use super::CommandStatus;
 
-// FIXME: This should return `io::Result<!>` but `!` is not stable yet.
 pub(super) fn exec_monitor(
     pty_follower: PtyFollower,
     command: Command,
     foreground: bool,
     backchannel: &mut MonitorBackchannel,
     mut file_closer: FileCloser,
     original_set: Option<SignalSet>,
-) -> io::Result<ExecOutput> {
+) -> io::Result<Infallible> {
     // SIGTTIN and SIGTTOU are ignored here but the docs state that it shouldn't
     // be possible to receive them in the first place. Investigate
     match SignalHandler::register(SIGTTIN, SignalHandlerBehavior::Ignore) {

src/exec/use_pty/parent.rs

@@ -182,7 +182,7 @@ pub(in crate::exec) fn exec_pty(
             file_closer,
             original_set,
         ) {
-            Ok(exec_output) => return Ok(exec_output),
+            Ok(exec_output) => match exec_output {},
             Err(err) => {
                 // Disable nonblocking assetions as we will not poll the backchannel anymore.
                 backchannels.monitor.set_nonblocking_assertions(true);