Idea: Ability to configure how default values are displayed in help messages

[mamekoro]

I want to customize the ... part of [default: ...] in help messages.

I have this code:

// clap = { version = "4.2.7", features = ["derive", "string"] }

use std::path::PathBuf;
use clap::Parser;

fn main() {
    println!("{:?}", Args::parse());
}

#[derive(Clone, Debug, Parser)]
pub struct Args {
    // `$CARGO_HOME` or `$HOME/.cargo`
    #[arg(long, default_value = default_cargo_home().into_os_string())]
    pub cargo_home: PathBuf,
}

fn default_cargo_home() -> PathBuf {
    if let Ok(path) = std::env::var("CARGO_HOME").map(PathBuf::from) {
        return path;
    }

    let home = std::env::var("HOME").expect("No such environment variable: HOME");
    [home.as_str(), ".cargo"].iter().collect()
}

cargo run -- --help outputs the following:
(USER is my username.)

Usage: idea [OPTIONS]

Options:
      --cargo-home <CARGO_HOME>  [default: /home/USER/.cargo]
  -h, --help                     Print help

However, what I want is the following:

Usage: idea [OPTIONS]

Options:
      --cargo-home <CARGO_HOME>  [default: $CARGO_HOME or $HOME/.cargo]
  -h, --help                     Print help

The former ([default: /home/USER/.cargo]) is dynamic because each user has a different username. I prefer more fixed help messages.

Also, the latter ([default: $CARGO_HOME or $HOME/.cargo]) shows that the value can be set via $CARGO_HOME, but the former does not.

It would be nice to be able to format default values in help messages, would you consider such a feature?

The relevant code can be found in clap_builder:

clap/clap_builder/src/output/help_template.rs

Line 784 in 475e254

spec_vals.push(format!("[default: {pvs}]"));

[epage]

Would it work to instead hide default values and then include what you want in the help you give for the argument?

[mamekoro]

Thanks. That approach seems to work well.