Why clap contains_id() always returns TRUE?

Question

This is the code (I'm using clap 3.2.20):

pub fn main() {
  let m = Command::new("foo")
    .arg(
      Arg::new("bar")
        .long("bar")
        .required(false)
        .action(ArgAction::SetTrue)
      )
    .get_matches();
  if m.contains_id("bar") {
    println!("Boom!");
  }
}

It prints Boom! either I provide --bar option or not. Why and how to fix?

Answer 1

Per the documentation of contains_id

NOTE: This will always return true if default_value has been set. ArgMatches::value_source can be used to check if a value is present at runtime.

Now you might object that you're not using default_value, however you're using ArgAction::SetTrue:

When encountered, act as if "true" was encountered on the command-line

If no default_value is set, it will be false.

Emphasis mine.

So ArgAction::SetTrue implies default_value=false.

Therefore bar will always have a value, there's no need to check whether it does. Just fetch the value.