clap: AppSettings::SubcommandsNegateReqs may cause panic in Clap::parse(), Clap::try_parse()

Clap version

3.0.0-beta.2

Where

https://docs.rs/clap/3.0.0-beta.2/clap/enum.AppSettings.html#variant.SubcommandsNegateReqs

What’s wrong

There’s no mention that this AppSetting can cause the Clap derive macro to panic on parse() and try_parse()

How to fix

use clap::{AppSettings, Clap, IntoApp, FromArgMatches};

#[derive(Debug, Clap)]
#[clap(setting(AppSettings::SubcommandsNegateReqs))]
struct ExOpts {
    
    req_str: String,

    #[clap(subcommand)]
    pub cmd: Option<SubCommands>,
}

#[derive(Debug, Clap)]
enum SubCommands{
    ExSub {
        #[clap(short, long, parse(from_occurrences))]
        verbose: u8
    },
}

fn main(){

    // We cant use Clap::parse or try_parse because we have SubcommandsNegateReqs enabled
    // this would cause a panic when Clap attempts to parse the req_str arg that isn't there
    // let opts = ExOpts::parse(); // panics
    // let opts = ExOpts::try_parse(); // panics 

    // Instead we need to check to see if a subcommand exists before we run from_arg_matches on ExOpts
    let matches = ExOpts::into_app().get_matches();
    if matches.subcommand_name().is_none() {
        // If no subcommand we can derive ExOpts
        let opts = ExOpts::from_arg_matches(&matches);
        println!("{:?}", opts);
    } else {
        // If subcommand we need derive the subcommands instead
        let cmd_opts = SubCommands::from_arg_matches(&matches);
        println!("{:?}", cmd_opts);
    }

}