Unable to Use Variable as default_value in Clap with Lifetime Issue

Question

I'm working on a Rust project where I'm using the Clap library to handle command-line arguments. I have a scenario where I want to set the default_value of a command-line argument based on a value calculated at runtime, but I'm encountering a lifetime issue.

Here's a simplified example of my code:

 let default_threads_name =  (num_cpus::get().saturating_sub(2).max(1)).to_string();

   let matches = Command::new("test")
    .version("1.0")
    .subcommand_required(true)
    .about("uploader")
    .arg(
        Arg::new("threadsHash")
            .long("threadsHash")
            .value_name("THREADS_HASH")
            .default_value(&*default_threads_name)
            .help("Number of threads for hashing"),
    )
    .get_matches();

The get_default_threads_name function calculates a value at runtime, and I want to use this value as the default_value for the "threadsHash" argument.

However, I'm encountering a lifetime issue because the borrowed value (default_threads_name) will expire by the time get_matches() returns the value. As a result, I'm unable to use the parsed arguments effectively.

Is there a way to work around this issue and set the default_value based on a runtime-calculated value without running into lifetime problems?

Any insights or solutions would be greatly appreciated!

Answer 1

Just pass default_threads_name itself and not a reference to it:

let matches = Command::new("test")
    .version("1.0")
    .subcommand_required(true)
    .about("uploader")
    .arg(
        Arg::new("threadsHash")
            .long("threadsHash")
            .value_name("THREADS_HASH")
            .default_value(default_threads_name)
            .help("Number of threads for hashing"),
    )
    .get_matches();

You need to enable the string feature of clap as well.