Using Cli default values, when using my crate as library

[BaumiCoder]

Hello,

I am new to Rust and want to use clap for a project that will be a lib and a bin crate.

When using my crate as library, it would be nice to be able to use the default values from the CLI, which are set with default_value_t for example. The standard way to get the default values for a struct in Rust seems to implement the Default trait. My expectation was that deriving this trait on a clap Args struct, leads to an implementation of this Trait using the default values for the CLI, but it does not.

Here is a small example project with one failing Integration Tests to show the usage as library crate and what default value I would expect:
https://codeberg.org/BaumiCoder/clap_default_values

How to use the CLI default values for implementing the Default Trait? Or is there another way to keep both defaults (for the Trait / library usage and for the CLI) in sync?

[epage]

Currently, this is unsupported. You can possibly work around it by having functions for each shared default and doing #[arg(default_value_t = foo_default())].

We were mirroring serde in its behavior with its default field attribute which uses the field's Default::default. serde also offers a default container attribute but that makes all fields defaulted which wouldn't be appropriate in clap. This is being tracked in #3116.

An unstable Rust feature allows for setting default values on fields directly. We'll need to evaluate how to integrate that into clap as that moves towards stabilization.

[BaumiCoder]

Okay, I have subscribed for the issue. For now, I may use some workaround which foo_default() functions.

Is there also a possibility to get the clap default with a method without setting it explicitly? Because when using ArgAction::SetFalse or ArgAction::SetTrue for Booleans, explicitly setting the clap default is a bit strange. (How would even a wrong setting like default_value_t = true together with ArgAction::SetTrue behave? Hopefully a compile time error 🤔)

[epage]

Default settings are stored in a CLI native format (OsString) and wouldn't work too well for you to pull from

(How would even a wrong setting like default_value_t = true together with ArgAction::SetTrue behave? Hopefully a compile time error 🤔)

SetTrue provides a set of defaults to create a flag and this would just be overriding one of those defaults.