FromField should warn or error if #[darling(attributes(...))] and #[darling(forward_attrs(...))] are both empty

[Mingun]

In #165 I faced with the problem, when I derive FromField for my struct and try to use it, but I've got nothing, because I've not specify #[darling(attributes(xml))]. Another derive that I tried before (if I remember correctly, FromAttributes) issues a compiler error about missing #[darling(attributes(...))].

It seems that without this attributes derive is useless, so it should be a compiler error if it is missed:

use darling::FromField;
use syn::{parse_macro_input, Data, DeriveInput, Fields};
use proc_macro::TokenStream;
use quote::ToTokens;

#[derive(Debug, Default, FromField)]
#[darling(default)]
//#[darling(attributes(xml))] // <<------------------------- missed 
struct FieldProps {
  /// Should be `true` when `#[xml(attribute)]` is present on a field and false otherwise:
  ///
  /// ```rust
  /// // ---> true
  /// #[xml(attribute)]
  /// field1: (),
  ///
  /// // ---> false
  /// field2: (),
  /// ```
  attribute: bool,
  // other attributes...
}

#[proc_macro_attribute]
pub fn quick_xml(_attr: TokenStream, item: TokenStream) -> TokenStream {
  let ast = parse_macro_input!(item as DeriveInput);

  if let Data::Struct(s) = &ast.data {
    if let Fields::Named(fields) = &s.fields {
      for f in &fields.named {
        let props = FieldProps::from_field(&f);

        println!("{}: {:?}", f.ident, props);
      }
    }
  }

  ast.into_token_stream().into()
}

successfully compiled

[TedDriggs]

Good catch! Technically, doing this works, it's just not very helpful with FromField.

Deriving FromDeriveInput and not passing attributes is actually useful, since you could use darling(supports(...)) for shape validation and the data field for automatic field conversion even if you don't care about any attributes - when I first built FromField I decided to allow it for consistency with FromDeriveInput. Now, however... I think it's reasonable to make it a compile error if both #[darling(attributes(...))] and #[darling(forward_attrs(...))] are empty.

P.S. If you didn't know about the FromDeriveInput trick, you can do the following:

#[derive(FromDeriveInput)]
#[darling(supports(struct_named))]
struct QuickXml {
    data: darling::ast::Data<(), FieldProps>
}

That would let you skip the if let pyramid in this example.