Passing a data structure as an args to a proc macro attribute

Question

I'm trying to create a macro that to use on a struct, in which you get another struct as an argument, then concatenate the structa fields. This the code of the macro

#[proc_macro_attribute]
pub fn cat(args: proc_macro::TokenStream, input: proc_macro::TokenStream) -> proc_macro:: TokenStream {
    let mut base_struct = parse_macro_input!(input as ItemStruct);
    let mut new_fields_struct = parse_macro_input!(args as ItemStruct);

    if let syn::Fields::Named(ref mut base_fields) = base_struct.fields {
        if let syn::Fields::Named(ref mut new_fields) = new_fields_struct.fields {
            for field in new_fields.named.iter() {
                base_fields.push(field.clone());
            }
        }
    }

    return quote! {
        #base_struct
    }.into();
}

Here's a basic example of what I want it to be used

struct new_fields{
    field: usize,
}

#[cat(new_fields)]
struct base_struct{
    base_field, usize,
}

However, it will just convert the word 'new_fields' into a TokenStream then fail to compile because it isn't a struct. Is there a way to 'pass' a struct to the macro as an argument without writing the code inside the the macro usage?

Answer 1

Is there a way to 'pass' a struct to the macro as an argument without writing the code inside the the macro usage?

No. Macros operate on tokens they are given and nothing else.

It's hard to give advice without knowing more detail, but the closest to what you've provided would be to have your macro generate the other struct, so that it knows the definition. You could perhaps pass it like this:

#[cat(
    struct new_fields{
        field: usize,
    }
)]
struct base_struct{
    base_field, usize,
}