Derive attribute: modify_fn(user_fn) where fn user_fn<T>(a: Arg) -> Arg generates user_fn::<T>(arg) for fields of type T

[marjakm]

clap changes

Added method to Arg

/// // Instead of this
/// let mut arg = Arg::new("test");
/// arg = func(arg);
///
/// // You can do this
/// let arg = Arg::new("test").modify_fn(func);
/// ```
pub fn modify_fn<F>(self, mod_fn: F) -> Self
where
    F: Fn(Self) -> Self
{
    (mod_fn)(self)
}

A free function

pub fn default_value_t<T>(arg: Arg) -> Arg
where
    T: Default + ToString
{
    let s = make_static_str(<T as Default>::default());
    arg.default_value(s).required(false)
}

clap_derive changes

modify_fn(_) attribute

#[derive(Parser, PartialEq, Debug)]
struct Opt {
    #[clap(modify_fn(clap::default_value_t))]
    arg: i32,
}

Expands to (snippet)

fn augment_args<'b>(__clap_app: clap::App<'b>) -> clap::App<'b> {
    {
        let __clap_app = __clap_app;
        let __clap_app = __clap_app.arg(
            clap::Arg::new("arg")
                .takes_value(true)
                .value_name("ARG")
                .required(true)
                .validator(|s| ::std::str::FromStr::from_str(s).map(|_: i32| ()))
                .modify_fn(clap::default_value_t::<i32>),
        );
        __clap_app
    }
}

Fixes #2991

[marjakm]

@epage Instead of making an Issue, it was easier to implement and show the idea.

By having the default_value_t function in clap crate, it can also be used by those that use the builder pattern instead of clap_derive.

This could also be more performant - the solution before was like a macro, that generated separate code for each Arg instance, but now it can generate the code for each T, but multiple args of the same T can use the same function.

[epage]

I'll take a look, thanks!

@epage Instead of making an Issue, it was easier to implement and show the idea.

The downside is that it couples the conversation to this one PR, so if this PR doesn't work out, its hard to keep it one place.

[epage]

Looking at this more, please create an issue and let's make sure we are on the same page on the need and how to solve that need over on that issue. This PR can serve as a tool to help us understand each other in that conversation.

[marjakm]

@epage I created the issue #2991

[epage]

Closing as we work out design in #2991