Extract/add public method Error.exit_code

[hartwork]

… and simplifies method Error.exit as a side effect.

The motivation for this pull request was to allow simplification of the code below (click to expand) where marked with XXX with future releases of clap.

// Copyright (c) 2023 Sebastian Pipping <sebastian@pipping.org>
// SPDX-License-Identifier: MIT

// ..snip..

// Matches the two internal constants from [clap_builder-4.3.1]/src/util/mod.rs  // XXX
const SUCCESS_CODE: i32 = 0;                                                     // XXX
const USAGE_CODE: i32 = 2;                                                       // XXX

fn main() {
    exit(middle_main(args_os()));
}

fn middle_main<I, T>(argv: I) -> i32
where
    // to match clap::Command.get_matches_from
    I: IntoIterator<Item = T>,
    T: Into<OsString> + Clone,
{
    let clap_result = command_line_parser::command().try_get_matches_from(argv);
    match clap_result {
        Ok(matches) => innermost_main(matches),
        Err(e) => {
            // This mimics clap::Error.exit minus the call to safe_exit
            let _ = e.print();
            if e.use_stderr() {                                                  // XXX
                USAGE_CODE                                                       // XXX
            } else {                                                             // XXX
                SUCCESS_CODE                                                     // XXX
            }                                                                    // XXX
        }
    }
}

fn innermost_main(matches: ArgMatches) -> i32 {
    // ..snip..
}

What do you think?

[epage]

For me, the main issue is that use_stderr is weird for callers to key off of for determining failure. This doesn't help with that when you want to change exit codes but does help if you are ok with the existing exit codes.

In terms of code bloat, this shouldn't add anything.

In terms of API bloat, looking through Error, this seems fine.

Probably the main question is whether to support ExitCode but that still seems like a relatively under-developed part of the stdlib to be trying to support here

[hartwork]

@epage thanks for the merge, cool! 🎉