How to automatically derive Default from clap annotated struct

[wiktor-k]

Hi,

I've got a clap annotated struct already:

#[derive(Parser, Debug)]
struct Args {
    /// Name of the person to greet
    #[arg(short, long. default_value = "John")]
    name: String,

    /// Number of times to greet
    #[arg(short, long, default_value_t = 1)]
    count: u8,
}

And, for test purposes, I'd like that struct to implement Default with values that I already specified in clap annotations. Is there a way to do that without this manual, repetitive, code?

impl Default for Args {
    fn default -> Self {
        Args { name: "John".into(), count: 1 } // <-- duplicating values that have been specified in clap
    }
}

Note that I want the Default::default to be automatically generated, not the other way around (which has been discussed in other questions).

Thank you for your help! 🙏

(I've done the search and most specific queries yield no results, OTOH searching for "default" brings many unrelated discussions but I might've overlooked something and if so: apologies in advance)

[epage]

There is not a way to automatically generate this and doing so would have enough caveats (performance, only working if all fields have defaults which we shouldn't push people towards) that I don't think we should do this as a first-party thing.

The performance issue stems from default_value. To turn those back into the field's type, we'd need to discover and run the value_parser. Ideally, these wouldn't fail but could in some cases.

For test purposes, you could hack together an even slower version, assuming all fields are defaulted:

use clap::Parser;

#[derive(Parser, Debug)]
struct Args {
    /// Name of the person to greet
    #[arg(short, long, default_value = "John")]
    name: String,

    /// Number of times to greet
    #[arg(short, long, default_value_t = 1)]
    count: u8,
}

impl Default for Args {
    fn default() -> Self {
        Args::try_parse_from(&["cmd"]).expect("defaults work")
    }
}

fn main() {
    dbg!(Args::default());
}