Make it easier to use clap validators with derive by generating arg associated constants?

[epage]

Please complete the following tasks

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

Clap Version

3.0.0-rc.4

Describe your use case

When deriving, if you define a group, deal with requires, conflicts, overrides, etc, then you need to reference the field name. You can type it out and App::debug_assert will fail if you get it wrong.

Example

#[derive(Debug, Parser)]
struct Opts {
    #[clap(long, conflicts_with = "hello-world")] // I thought it should be hello_world
    one: Option<String>,
    #[clap(long)]
    hello_world: Option<String>,
}

Describe the solution you'd like

What if instead we generated something like

impl Opt {
    const ONE_CLAP_ARG: &str = "one";
    const HELLO_WORLD_CLAP_ARG: &str = "hello-world";
}

So you could do

#[derive(Debug, Parser)]
struct Opts {
    #[clap(long, conflicts_with = Opts::HELLO_WORLD_CLAP_ARG)]
    one: Option<String>,
    #[clap(long)]
    hello_world: Option<String>,
}

I'm assuming this will work with macro expansion

Alternatives, if applicable

Provide helpers to avoid it, like #3165

Additional Context

This was inspired by #2475

[epage]

Benefits:

• Move errors to compile time

Downsides

• I personally find it confusing accessing items generated by a macro that are unrelated to the trait
• Potential name conflicts