builtin commands ignore redirects when printing man pages

[sd-collins]

fish, version 3.7.0-1382-g54bc19691
OS: Arch Linux, kernel: 6.6.14-1-lts
Terminal: Konsole 23.08.4-2
Same behaviour with/without third-party customisations.

Noticed when debugging a local test failure in builtinbuiltin.fish. The test script does this:

fish-shell/tests/checks/builtinbuiltin.fish

Lines 13 to 15 in 54bc196

	builtin -- -q &| grep -q "builtin - run a builtin command\|fish: builtin: missing man page"
	echo $status
	#CHECK: 0

This works when the man page is missing, but fails when it is present, even though the match string accounts for this case.
The failure is caused by the function that prints the man page:

fish-shell/src/builtins/shared.rs

Lines 544 to 568 in 54bc196

	pub fn builtin_print_help_error(
	parser: &Parser,
	streams: &mut IoStreams,
	cmd: &wstr,
	error_message: &wstr,
	) {
	// This won't ever work if no_exec is set.
	if no_exec() {
	return;
	}
	let name_esc = escape(cmd);
	let mut cmd = sprintf!("__fish_print_help %ls ", &name_esc);
	let mut ios = IoChain::new();
	if !error_message.is_empty() {
	cmd.push_utfstr(&escape(error_message));
	// If it's an error, redirect the output of __fish_print_help to stderr
	ios.push(Arc::new(IoFd::new(STDOUT_FILENO, STDERR_FILENO)));
	}
	let res = parser.eval(&cmd, &ios);
	if res.status.normal_exited() && res.status.exit_code() == 2 {
	streams
	.err
	.append(wgettext_fmt!(BUILTIN_ERR_MISSING_HELP, name_esc, name_esc));
	}
	}

Notice towards the end of the function that if the man page is missing, we simply append the error message to stderr:

    if res.status.normal_exited() && res.status.exit_code() == 2 {
        streams
            .err
            .append(wgettext_fmt!(BUILTIN_ERR_MISSING_HELP, name_esc, name_esc));
    }

All good so far. But taking a look at the code running the man page, we do something entirely different:

    let mut ios = IoChain::new();
    if !error_message.is_empty() {
        cmd.push_utfstr(&escape(error_message));
        // If it's an error, redirect the output of __fish_print_help to stderr
        ios.push(Arc::new(IoFd::new(STDOUT_FILENO, STDERR_FILENO)));
    }
    let res = parser.eval(&cmd, &ios);

Since we're creating a new IoChain, any existing redirects held in streams.io_chain are ignored, leading to the help text being printed to stdout (or stderr if there is help text) regardless of whether it is being redirected. This causes the test to fail (and as a side note, this behaviour is pretty inconsistent).

I believe this also affects other builtins, for example read --help &> /dev/null will still print the man page to stdout.