Clap - set default value based on another flag

• mode is required argument.
• The value of log_level is based on the mode.
• We can also set the log_level manually.

$ cargo run -q -- --mode devel
Args: Cli { mode: Devel, log_level: 1 }

$ cargo run -q -- --mode test
Args: Cli { mode: Test, log_level: 2 }

$ cargo run -q -- --mode release
Args: Cli { mode: Release, log_level: 0 }

$ cargo run -q -- --mode release --log-level 10
Args: Cli { mode: Release, log_level: 10 }

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

#[derive(Debug, Clone, Copy, ValueEnum)]
enum Mode {
    Devel,
    Test,
    Release,
}

#[derive(Debug, Parser)]
struct Cli {
    #[arg(long)]
    mode: Mode,

    #[arg(
        long,
        default_value_if("mode", ArgPredicate::Equals(OsStr::from("devel")), "1"),
        default_value_if("mode", ArgPredicate::Equals(OsStr::from("test")), "2"),
        default_value_t = 0
    )]
    log_level: u8,
}

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

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