clap_derive: Default value support for Vec of ValueEnums

[emersonford]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

3.2.10

Describe your use case

Suppose you have the following:

#[derive(clap::ValueEnum, PartialEq, Debug, Clone)]
enum ArgChoice {
    Foo,
    Bar,
}

#[derive(Parser, PartialEq, Debug)]
struct Opt {
    #[clap(value_enum, value_parser)]
    arg: Vec<ArgChoice>,
}

providing default values for arg doesn't feel very ergonomic to me. As far as I'm aware, you have the following options:

1. #[clap(value_enum, value_parser, default_value="foo,bar", value_delimiter=',')] as shown in fix: multiple default values combined with ArgEnum / PossibleValues #3541
   • Doesn't feel super great to me since you're forced into using value_delimiter and can't easily set the default to "all variants". Also, if you ever use rename_all, you'd have to ensure default_value also gets changed.
2. default_values(...) with something like

   ArgChoice::value_variants().iter().map(|val| val.to_possible_value().unwrap().get_name()).collect::<Vec<_>>().as_slice()

   or

   &[ArgChoice::Foo.to_possible_value().unwrap().get_name(), ArgChoice::Foo.to_possible_value().unwrap().get_name()]

   • Requires a lot of boiler plate and repeated code.

Describe the solution you'd like

Fixing default_value_t so you can do the following:

#[derive(clap::ValueEnum, PartialEq, Debug, Clone)]
enum ArgChoice {
    Foo,
    Bar,
}

#[derive(Parser, PartialEq, Debug)]
struct Opt {
    #[clap(value_enum, value_parser, default_value_t = vec![ArgChoice::Foo])]
    arg: Vec<ArgChoice>,
}

and

#[derive(clap::ValueEnum, PartialEq, Debug, Clone)]
enum ArgChoice {
    Foo,
    Bar,
}

#[derive(Parser, PartialEq, Debug)]
struct Opt {
    #[clap(value_enum, value_parser, default_value_t = ArgChoice::value_variants().to_vec())]
    arg: Vec<ArgChoice>,
}

Alternatives, if applicable

I could see an argument for adding a default_values_t attr, but since default_value_t doesn't work to begin with for Vec<impl ValueEnum>, I feel like we might as well fix default_value_t (plus I personally find it more intuitive, if I used the "typed" version for a Vec, I should be able to pass a Vec).

Additional Context

default_value_t doesn't work in any way:

#[derive(Parser, PartialEq, Debug)]
struct Opt {
    #[clap(
        value_enum, 
        value_parser, 
        default_value_t = ArgChoice::Foo,
    )]
    arg: Vec<ArgChoice>,
}

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
  --> src/main.rs:14:27
   |
14 |         default_value_t = ArgChoice::Foo,
   |                           ^^^^^^^^^^^^^^ expected struct `Vec`, found enum `ArgChoice`
15 |     )]
16 |     arg: Vec<ArgChoice>,
   |          -------------- expected due to this
   |
   = note: expected struct `Vec<ArgChoice>`
                found enum `ArgChoice`

#[derive(Parser, PartialEq, Debug)]
struct Opt {
    #[clap(
        value_enum, 
        value_parser, 
        default_value_t = vec![ArgChoice::Foo],
    )]
    arg: Vec<ArgChoice>,
}

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the trait bound `Vec<ArgChoice>: ArgEnum` is not satisfied
  --> src/main.rs:14:9
   |
14 |         default_value_t = vec![ArgChoice::Foo],
   |         ^^^^^^^^^^^^^^^ the trait `ArgEnum` is not implemented for `Vec<ArgChoice>`
   |
   = help: the trait `ArgEnum` is implemented for `ArgChoice`

so changing default_value_t's behavior here for a Vec of ValueEnums wouldn't be a breaking change.

[emersonford]

documenting this here from the PR:

I realized I could apply this for all Vec, not just a Vec.

so I added another commit to the PR to fix default_value_t for all Vec types

[benny-n]

I realized that I need this feature today and this design is exactly what I had in mind!
Looking forward to seeing this merged 🙂