How are the positional arguments to struct Cli matched to the fields _in order_ ?

[iampi31415]

Somewhat new to Rust and Clap. I find the tool superb so far, and can only thank you for it.

There is an example I didn't understand from the tutorial, and I wonder if you could explain it.

It just looks a bit magical an aspect of the [positional arguments]((https://docs.rs/clap/latest/clap/_derive/_tutorial/index.html#positionals). For example,

use clap::Parser;

#[derive(Parser)]
#[command()]
struct Cli {
    name: Option<String>,
    surname: Option<String>,
}

fn main() {
    let cli = Cli::parse();
    if let Some(name) = cli.name {
        println!("Name: {name}");
    }
    if let Some(surname) = cli.surname {
        println!("Surname: {surname}");
    }
}

So in cargo run -- Arin Mand it assigns "name: Arin", "surname:Mand". They are assigned from the position in the CLI, to the position in the struct as read by a human.
Inverting it:

struct Cli {
    surname: Option<String>,
    name: Option<String>,
}

It reverses the print to Mand Arin. Then any new named arguments with the #[arg(..)] are handled independently which is good.

This is probably a basic question, but I don't quite see how do you obtain the names of the struct in order they are declared in the text ?! Just a small conceptual explanation would be enough for me, but got me stuck pondering that.

[epage]

This is probably a basic question, but I don't quite see how do you obtain the names of the struct in order they are declared in the text ?! Just a small conceptual explanation would be enough for me, but got me stuck pondering that.

#[derive(Parser)] activates a compiler plugin, clap_derive and passes a copy of the Rust Tokens for the entire struct to that plugin. We then iterate on them in order to provide this.

[iampi31415]

Thank you