How to get the ValueSource for an argument?

Question

I am using Clap (derive) v4.x for a Rust CLI application. The basics are working properly for command line arguments, environment variable values, and default values.

Next, for each match, I want to determine the ValueSource (default, envvariable, command line), but I cannot figure out how to get this information using a derived subcommand (i.e., my ContainerCommand in the code, below). I hope someone can give me a hint or a nudge in the right direction.

I need to implement hierarchical evaluation of the potential configuration data for the app: cmdline > envvar > config file > defaults.

My goal is to deserialize config information from a toml file. Copy the data from clap into a Config struct. For any items in the Config struct that are None or came from Defaults, I will replace the value with what is found from the toml file's data, if it exists. Otherwise, the default value will be set.

I know that there are excellent packages (Figment, Config-rs, Twelf, …) that implement hierarchical configuration processing, but each one of them offers unique challenges/issues when I've tried to use them in our application. Hence, I want to use the simple solution that I've described.

Here is the strawman app that I am using to figure things out:

/// This is the entry point for the application.
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let app = App::parse();
    dbg!(&app);
    
    match &app.command {
        Command::Container(c) => {
             //let matches = ContainerCommand::command().get_matches();
    //         if matches.contains_id("name") {
    //             println!("{:?}", c.name);
    //             match matches.value_source("name").unwrap() {
    //                 ValueSource::DefaultValue => { println!("Is the default value")}
    //                 ValueSource::EnvVariable => { println!("Is the envvar value")}
    //                 ValueSource::CommandLine => { println!("Is the commandline value")}
    //                 _ => { println!("Unknown source for the value...")}
    //             }
    //         }
         }
     }

    Ok(())
}

/// The definition of the command line and its arguments
#[derive(Debug, Parser, Serialize)]
#[command(author, version, about, long_about = None, next_line_help = true, propagate_version = true)]
pub struct App {
    #[command(flatten)]
    global_opts: GlobalOpts,

    #[command(subcommand)]
    command: Command,
}

#[derive(Debug, Args, Serialize)]
struct GlobalOpts {
    /// config_path is the path to a configuration (.toml) file, which defaults to the current directory.
    #[arg(short = 'c', long, global = true, default_value = "config.toml")]
    config_path: std::path::PathBuf,
}

#[derive(Debug, Subcommand, Deserialize, Serialize)]
enum Command {
    #[command(about = "The hash container command determines the base64 binary MD5 hash for each blob in a container.", long_about = None)]
    /// Determines the base64 binary MD5 hash for each blob in a container
    Container(ContainerCommand),
}

/// The definition of the command line and its arguments
#[derive(Parser, Debug, Deserialize, Serialize)]
struct ContainerCommand {
    /// The name of a client
    #[arg(short = 'n', long, env("NAME"), default_value = "kdev")]
    name: Option<String>,
}

Answer 1

You can't use parse as that already does all the parsing and throws away the information you need. Instead you have to use App::command().get_matches():

    use clap::CommandFactory;
    let matches = App::command().get_matches_from(["self", "-c", "foo", "container"]);
    if let Some((_subcommand_name, subcommand_matches)) = matches.subcommand() {
        match subcommand_matches.value_source("name") {
            Some(ValueSource::DefaultValue) => println!("Is the default value"),
            Some(ValueSource::EnvVariable) => println!("Is the envvar value"),
            Some(ValueSource::CommandLine) => println!("Is the commandline value"),
            Some(_) => println!("Unknown source for the value..."),
            None => (),
        }
    }

and you can later get the App using FromArgMatches:

    use clap::FromArgMatches;
    let app = App::from_arg_matches(&matches).unwrap();