clap_derive: Equivalent of #[arg(value_parser = ...)] for custom parsing of subcommands

[bbb651]

I'm trying to make a custom format (optional json output) for an explicit help subcommand rather than the normal --help and implicit help message when not specifying a subcommand. I've got it mostly working with:

#[derive(Subcommand)]
pub enum Msg {
    Action {
        #[command(subcommand)]
        action: SubcommandOrHelp<Action>,
    },
    // ...
}

#[derive(Debug, Subcommand)]
#[command(disable_help_subcommand(true))]
pub enum SubcommandOrHelp<T: Subcommand> {
    #[command(flatten)]
    Subcommand(T),
    Help,
}

This works as expected, but my problem is that all the Help command does is print the available subcommands in a different format to stdout and then exit, and my Msg which is deeply nested is later passed to other modules which makes SubcommandOrHelp very leaky (by that point I would've already exited the program, so I have to put unreachable()!s). The other problem is that handling it is verbose and requires getting the exact path of the subcommand (and I want to reuse this for multiple different subcommands):

let cli = Cli::parse();
let command = Cli::command();
// ...
match cli.subcommand {
    Sub::Msg { msg, json } => {
            if let Msg::Action {
        action: SubcommandOrHelp::Help,
    } = msg
    {
        // TODO: Non-json output
        #[derive(Serialize)]
        struct ActionJson<'a> {
            name: &'a str,
            description: Option<String>,
        }
        let actions = command
            .find_subcommand("msg")
            .unwrap()
            .find_subcommand("action")
            .unwrap()
            .get_subcommands()
            .map(|action| ActionJson {
                name: action.get_name(),
                description: action.get_about().map(|about| format!("{about}")),
            })
            .collect::<Vec<_>>();
        serde_json::to_writer(std::io::stdout(), &actions).unwrap();
        return Ok(());
    };
    // ...
}

I think a custom subcommand value parser will solve both of these issues, does anyone know if something like that exists or if there's an alternative approach that might work?

[epage]

Custom subcommand actions have been brought up from time to time but no serious design work has been done. We don't even have an issue for it yet. One of the complexities to that ArgAction controls what happens once an argument has been fully parsed and some would want similar for subcommands (e.g. clap-nested but Help needs effectively act as an external_subcommand and process arguments in a custom way.