Clap - set default value if other argument provided

• default_value_if

• ArgPredicate

• IsPresent

• This is a similar example but now we say that if the user provides a name for the logfile then we'll automatically turn on logging to that file.

• This might be more logical than the previous one, but I am not sure I like this either.

use clap::builder::ArgPredicate;
use clap::Parser;

#[derive(Debug, Parser)]
struct Cli {
    #[arg(long, default_value_if("log_file", ArgPredicate::IsPresent, "true"))]
    log_to_file: bool,

    #[arg(long, default_value = "my.log")]
    log_file: String,
}

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

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

$ cargo run -q
Args: Cli { log_to_file: false, log_file: "my.log" }

$ cargo run -q -- --log-to-file
Args: Cli { log_to_file: true, log_file: "my.log" }

$ cargo run -q -- --log-file other.log
Args: Cli { log_to_file: true, log_file: "other.log" }

$ cargo run -q -- --log-to-file  --log-file other.log
Args: Cli { log_to_file: true, log_file: "other.log" }