Enum support

[Deaths-Door]

I've been working on supporting enum variants (#23) (currently only named enum variants have any progress) with the #[builder] proc-macro in bon. While I've made progress, there are a couple of issues and limitations I'm stuck on and would appreciate some guidance/help.

Issues:

• Double struct generation: The generated code seems to define the generated EnumVariantBuilder struct twice . The builder implementation, however, is only defined once.
• Original enum disappearance: When placing the #[builder] macro on an enum, the original enum definition disappears from the expanded code. This can be bypassed by defining the enum twice, but this is a hack.

Example Code

#[builder]
enum Expression {
  None,
  Binary {
    operation: char,
    left: u32,
    right: u32,
  }
}

This code results in the strange behavior mentioned above. Defining the enum twice allows compilation:

#[builder]
enum Expression {
  None,
  Binary {
    operation: char,
    left: u32,
    right: u32,
  }
}

enum Expression {
  None,
  Binary {
    operation: char,
    left: u32,
    right: u33,
  }
}

Limitations

• No generics support currently: The current implementation doesn't support generics for enums. I'm unsure how to approach defining minimal generics (mainly bounds) for the enum variants. My idea is to introduce a new attribute like bon(bound = "..."), similar to serde(bound = "..."). (which is obviously up for discussion)

• Customization of the generated struct is disabled : Currently, customizing the generated struct is not supported. This is due to the StructInputParms struct encountering an error when using from_meta::from_list on enum variants, as it returns an error, which has a gist of unknown field Binary.

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