Allow accessing the env name and default value when iterating over the arguments

[shepmaster]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Describe your use case

I want to offer the ability to quickly generate a placeholder .env file for developers working on my projects. This information is contained with the App struct via the name / help text / environment variable name. However, I cannot access the environment variable or default value.

dotenv is a crate that allows specifying an .env file containing environment variables in a file. This allows developers to have a semi-persistent way to store and document environment variables needed to run an application.

Clap allows specifying environment variables as fallback sources for command line options. This includes the environment variable names, documentation about them, default values, possible values, etc.

Describe the solution you'd like

I want to reuse the definitions we've already given to Clap to generate a skeleton .env file:

use clap::{IntoApp, Clap};

#[derive(Clap)]
struct Config {
    #[clap(long, default_value = "world", env = "NAME")]
    /// The name to use
    name: String,
}

fn main() {
    let app: clap::App = Config::into_app();

    for arg in app.get_arguments() {
        println!("# {}", arg.get_about().unwrap_or(""));
        println!("# {}", arg.get_possible_values().unwrap_or(&[]).join(", "));
        println!("# {} = {}", arg.get_env(), arg.get_default_value());
        // Does not exist     ^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^
    }
}

This will generate the output

# The name to use
# 
# NAME = world

A developer starting on the project can direct that output to a .env file and edit it as needed.

Alternatives, if applicable

No response

Additional Context

Ideally in a stable release of Clap, but in a beta release would be cool too!

[pksunkara]

Need more description and examples. Not really sure what you are asking here.

[shepmaster]

Updated.