当使用structopt时，如何测试用户是否提供了选项，或者它是否来自默认值？

use structopt::StructOpt;

#[derive(Debug, StructOpt)]
struct Cli {
    #[structopt(short, long)]
    input: Option<String>,
    
    #[structopt(short, long)]
    flag: bool,
    
    #[structopt(short, long)]
    marker: bool,
}

fn main() {
    let args = Cli::from_args();
    
    if args.input.is_some() {
        println!("Input option provided: {:?}", args.input.unwrap());
    } else {
        println!("Input option not provided");
    }
    
    if args.flag {
        println!("Flag option provided");
    } else {
        println!("Flag option not provided");
    }
    
    if args.marker {
        println!("Marker option provided");
    } else {
        println!("Marker option not provided");
    }
}