Attributes

#[structopt(...)] attributes fall into two categories:

• structopt's own magical methods.

  They are used by structopt itself. They come mostly in attr = ["whatever"] form, but some attr(args...) also exist.

• raw attributes.

  They represent explicit clap::Arg/App method calls. They are what used to be explicit #[structopt(raw(...))] attrs in pre-0.3 structopt

Every #[structopt(...)] attribute looks like comma-separated sequence of methods:

#[structopt(
    short, // method with no arguments - always magical
    long = "--long-option", // method with one argument
    required_if("out", "file"), // method with one and more args
    parse(from_os_str = path::to::parser) // some magical methods have their own syntax
)]

#[structopt(...)] attributes can be placed on top of struct, enum, struct field or enum variant. Attributes on top of struct or enum represent clap::App method calls, field or variant attributes correspond to clap::Arg method calls.

In other words, the Opt struct from the example above will be turned into this (details omitted):

#![allow(unused_variables)]
fn main() {
use structopt::clap::{Arg, App};
App::new("example")
    .version("0.2.0")
    .about("An example of StructOpt usage.")
.arg(Arg::with_name("debug")
    .help("Activate debug mode")
    .short("debug")
    .long("debug"))
.arg(Arg::with_name("speed")
    .help("Set speed")
    .short("v")
    .long("velocity")
    .default_value("42"))
// and so on
;
}