That macro reminds me a lot of the macros of old times from before the availability of proc macros, which would manually parse Rust code to rely on their featur

That macro reminds me a lot of the macros of old times from before the availability of proc macros, which would manually parse Rust code to rely on their features. It's probably done that way because they don't want proc macros to be used for the Rust code. I guess this is because of the fear of the heavy dependency tree of stuff like syn, but you don't even need syn for simple macros, you can also parse them manually instead.