在 clap 中动态生成带有格式化描述的子命令

Question

I want to generate subcommands for clap based on a HashMap content. To do that, I'm iterating over the items to create new App instances. The problem is that App.about() requires &str as an input, and I can't make one that lives long enough from the format!() macro.

Here's an example ( playground ):

use clap::App;
use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    map.insert("key", "value");
    map.insert("key2", "value2");
    map.insert("key3", "value3");

    let mut app = App::new("Test").version("1.0");

    for (key, value) in map.iter() {
        let about = format!("Value: {}", value);
        app = app.subcommand(App::new(*key)).about(about.as_str());
    }

    app.get_matches();
}

Error message:

error[E0597]: `about` does not live long enough
  --> src/main.rs:14:52
   |
14 |         app = app.subcommand(App::new(*key)).about(about.as_str());
   |                                                    ^^^^^ borrowed value does not live long enough
15 |     }
   |     - `about` dropped here while still borrowed
16 | 
17 |     app.get_matches();
   |     --- borrow later used here

How can I pass the formatted string to the about() method?

Answer 1

First of all, it looks like you have an unrelated mistake, which I'll mention just to reduce confusion about code sample correctness. I haven't used clap myself, but it looks like this is overwriting the app's about repeatedly

app = app.subcommand(App::new(*key)).about(about.as_str());

and you probably meant

app = app.subcommand(App::new(*key).about(about.as_str()));

to apply it to the subcommand instead.

---

The problem here is that App doesn't want to own the strings, so there needs to be another object that does. One approach would be to construct a map that contains the format! ted strings (and thus owns them):

    let abouts: HashMap<String, String> = map.iter()
        .map(|(key, value)| (key.to_string(), format!("Value: {}", value)))
        .collect();
    for (key, about) in abouts.iter() {
        app = app.subcommand(App::new(key.as_str()).about(about.as_str()));
    }

The map abouts must live as long as app .

If you're only going to do this once in your program, then you can just leak the memory format! allocated:

    for (key, value) in map.iter() {
        let about = &*Box::leak(format!("Value: {}", value).into_boxed_str());
        app = app.subcommand(App::new(*key).about(about));
    }

If you want to be able to reclaim the memory but don't want to have to deal with constructing a structure like abouts above, because your actual problem is more complex, you can use the typed_arena crate to provide a place to allocate any number of strings and keep them all alive:

use typed_arena::Arena;

...

    let arena = Arena::new();
    for (key, value) in map.iter() {
        let about = arena.alloc(format!("Value: {}", value)).as_str();
        app = app.subcommand(App::new(*key).about(about));
    }

Like in the abouts solution, you need to keep arena around as long as you keep app around.