I see. The parts of this header file that cannot be trivially ported to Zig are those that involve conditional evaluation at runtime. You can use comptime to include this sort of sub-language in the language (and this is how e.g. std.fmt works), but I'm not sure if one can achieve the level of tight coupling with the outer language that you have.

It's an explicit goal of the language that you can tell what the local control flow is by reading the code, but of course one is free to run the C preprocessor as part of the build if one disagrees.

I don't think C++ constexpr/consteval can be used to write things like generic json serializers and deserializers, which Zig does in the standard library using comptime, but I'm not sure.