[modular] User-driven values (defaults, env vars, implied values, etc)

[epage]

With clap today, each value source needs to be hard coded, including

• default_value
• default_missing_value
• default_value_if
• env

When new use cases arise, users either have to

• Hard code the values
• Get a new feature approved and contributed which runs up against Reduce compilation time #2037 / Binary size could be smaller #1365

It would help if clap had an API for users to provide their own systems for this. See also #3476.

Example unsupported use cases

• Have an argument imply others and set them? #4788 implied values
  • Implied values should have the same value source as the caused value
  • Ideally we let users leverage an actions default value / default missing value
• Add method to parse environment variables from custom source #4607: custom env variable source for wasm
• Add simple prompts #1634

default_value and default_missing_value are integral to Actions, so I'm thinking we leave those as built-in

Requirements

• Know the source for existing relevant arg values
• Able to set the source for value
• Able to augment the help description of a specific argument with details
• Ideally help uses with ValueSource precedence
• Ideally leverage built-in Action behavior
• Ideally not so unergonomic to be unuseful

Tools

• Plugin API to allow storing arbitrary data on Command/Arg
• Blanket impl of a trait for functions
• Into to allow custom plugin data to be converted to the plugin type

[epage]

Idea

trait ValueSource {
    /// Fill in additional values
    fn fill_values(&self, cmd: &Command, values: &mut ...) {}
    /// Augment an arguments help output with additional information, like defaults, env variables and their values, etc
    fn arg_hint(&self, cmd: &Command, arg: &Arg)  -> Option<Hint> {}
}

Ideally, we allow a user to define a source on an Arg and automatically adapt it to be for the Command, imagine if:

struct Env {
    name: Str
}

struct CommandEnv {
    id: Id
    value: Env
}

impl From<(&'_ Arg, Env)> for CommandEnv {
    fn from(other: (&'_ Arg, Env)) -> Self {
        Self { id: other.0.get_id(), value: other.1 }
    }
}