Clap - set default value if other flag provided

• default_value_if

• ArgPredicate

• Equals

• OsStr

• If log_to_file is true then log_file will get a default value.

• Actually I am not sure this is such a good example for default_value_if as setting the default even if the log_to_file is False would not be a problem. The code just would not use it.

• ArgPredicate

• default_value_if

use clap::builder::{ArgPredicate, OsStr};
use clap::Parser;

#[derive(Debug, Parser)]
struct Cli {
    #[arg(long)]
    log_to_file: bool,

    #[arg(
        long,
        default_value_if("log_to_file", ArgPredicate::Equals(OsStr::from("true")), "my.log")
    )]
    log_file: Option<String>,
}

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

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

$ cargo run -q
Args: Cli { log_to_file: false, log_file: None }

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

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

$ cargo run -q -- --log-file other.log
Args: Cli { log_to_file: false, log_file: Some("other.log") }