How do I use default_value_t with an Enum?

[crawford]

I'm trying to specify the default value for an enum using default_value_t but compilation fails saying that the enum "cannot be formatted with the default formatter". Here's a simplified version of what I'm trying to do (Playground):

#[derive(clap::Parser)]
struct Opts {
    #[clap(default_value_t = AorB::A)]
    aorb: AorB,
}

#[derive(Clone, clap::ValueEnum)]
enum AorB {
    A,
    B,
}

I can work around the issue by using default_value = "a", but I'd like to be able to check as much as possible during compilation. Somewhat interestingly, if I don't specify a default, the help output still figures out how to print the enum, so I'm guessing this functionality is provided by the ValueEnum macro.

[epage]

The core error message is

AorB doesn't implement std::fmt::Display

This is needed to render the default value in the help. You can use the ValueEnum trait to implement this: https://github.com/clap-rs/clap/blob/master/clap_builder/src/util/color.rs#L82

[crawford]

Thanks for the quick response! I thought that functionality would be automatically generated by the macro, but it's easy enough to implement. Thanks for your help.

[epage]

I'm not a fan of macros doing "magic" things behind users backs. For instance, a user might want to #[value(skip)] a field but want it rendered.

I am considering adding a ValueEnumDisplay derive (or whatever would be a good name to clarify this is a specialized Display derive)

[crawford]

Ah, that makes sense. I hadn't considered the skip case. (Totally agree with you about avoiding magic.)