fields_from_struct.rs

use proc_macro::TokenStream;
use quote::{format_ident, quote};
use syn::{parse_macro_input, Data, DeriveInput, Fields};

#[proc_macro_derive(FieldsFromStruct)]
pub fn fields_from_struct_derive(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);

    let visibility = &input.vis;
    let enum_name = format_ident!("{}Fields", input.ident);
    let fields = if let Data::Struct(data) = input.data {
        match data.fields {
            Fields::Named(fields) => fields.named,
            _ => panic!("Only named fields are supported"),
        }
    } else {
        panic!("Only structs are supported");
    };

    let enum_variants = fields.into_iter().map(|f| f.ident).collect::<Vec<_>>();

    let expanded = quote! {
        #[allow(non_camel_case_types)]
        #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
        #visibility enum #enum_name {
            #(#enum_variants),*
        }
    };

    TokenStream::from(expanded)
}