How can I access a runtime-defined variable in stuctopt definitions?

Question

I would like to be able to use the value of a variable (or better yet, the return of a function(arg)) as the about string for a CLI program defined using structopt. The end goal is a fully localized CLI that detects the system language or an ENV var and loads up localized strings that get baked into the --help message, etc.

By default it uses the documentation comment:

/// My about string
#[derive(StructOpt)]
struct Cli {}

I've found I can pass a manually entered string instead:

#[derive(StructOpt)]
#[structopt(about = "My about string")]
struct Cli {}

That's one step closer, but what I really want to do is pass a variable:

let about: &str = "My about string";
#[derive(StructOpt)]
#[structopt(about = var!(about))]
struct Cli {}

That last block is pseudo-code because I don't know what syntax to use to achieve this. In the end, I'll need more than just a single string slice, but I figured that was a place to start.

How can I pass through values like this to structopt? Do I need to access the underlying clap interfaces somehow?

Answer 1

StructOpt just adds a derive macro and a corresponding trait over clap. The clap crate has a function to set the about message at runtime, so we just need to add that. If we look at how from_args works we can see that it creates the clap App struct before turning it into the user-defined struct.

Thus, to do what you want:

use structopt::StructOpt;

fn get_localized_about() -> &'static str {
    "localized string"
}

#[derive(Debug, StructOpt)]
struct Foo {}

fn main() {
    let foo = Foo::from_clap(&Foo::clap().about(get_localized_about()).get_matches());
    println!("{:#?}", foo);
}