what2watch/optional_optional_user/src/lib.rs

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, ItemStruct};

#[proc_macro_derive(OptionalOptionalUser)]
pub fn derive_optional_optional_user(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as ItemStruct);
    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
    let name = &input.ident;
    let has_user = input
        .fields
        .iter()
        .find(|f| {
            if let Some(ref ident) = f.ident {
                ident == "user"
            } else {
                false
            }
        })
        .is_some();

    let (use_any, user_is_option_user) = if has_user {
        (
            quote!(
                use ::std::any::Any;
            ),
            quote!(
                ::std::any::TypeId::of::<::std::option::Option<crate::User>>()
                    == self.user.type_id()
            ),
        )
    } else {
        (quote!(), quote!(false))
    };

    let output = quote!(
        impl #impl_generics #name #ty_generics #where_clause {
            pub fn has_optional_user(&self) -> bool {
                #use_any
                #user_is_option_user
            }
        }
    );

    TokenStream::from(output)
}