Clap: Only Show Arg as Environment Variable

help
1

This is kind of a weird question, but I'm wondering if anyone knows whether this is possible. I want to display in my program's help text certain environment variables that can influence the behavior of the program that don't correspond to a CLI argument.

For example, consider the RUST_LOG variable: it will be used by the telemetry library internally and has no value/effect when it is specified as a CLI argument. Nevertheless, I'd like to make the users aware of this value when help is invoked so that they know it exists.

use clap::Parser;

#[derive(Debug, Clone, Parser)]
struct Args {
    /// Comma-delimited log directives to be used for filtering telemetry
    #[clap(env = "RUST_LOG")]
    rust_log: Option<String>,
}

fn main() {
    let args = Args::parse();

    println!("Parsed args: {args:?}");
}

Output:

Usage: clap-demo [RUST_LOG]

Arguments:
  [RUST_LOG]  Comma-delimited log directives to be used for filtering telemetry [env: RUST_LOG=]

Options:
  -h, --help  Print help

I don't see a way within clap to prevent this from showing as an argument but showing as an environment variable. I've tried the following, and it hides the argument altogether and displays nothing:

#[derive(Debug, Clone, Parser)]
struct Args {
    /// Comma-delimited log directives to be used for filtering telemetry
    #[clap(hide(true), hide_env(false), env = "RUST_LOG")]
    rust_log: Option<String>,
}

Is what I'm seeking to do possible at all with latest clap? I might have to defer to a custom text block otherwise.

2

After trying various things as described above, I filed a feature request to clap's GitHub issues: Hide Argument as CLI, Show as Environment Variable · Issue #6039 · clap-rs/clap · GitHub

I've contributed to clap in the past and I may look into doing this if the maintainers are on board with a specific design decision.