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 name = &input.ident;
    let has_user = input
        .fields
        .iter()
        .find(|f| {
            if let Some(ident) = f.ident.clone() {
                ident == "user"
            } else {
                false
            }
        })
        .is_some();

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

    let output = quote!(
        impl #name {
            fn has_optional_user(&self) -> bool {
                use ::std::any::Any;
                #has_user #user_is_option_user
            }
        }
    );

    TokenStream::from(output)
}