fun compile_header_attribute_structs
	do
		if modelbuilder.toolcontext.opt_no_union_attribute.value then
			self.header.add_decl("typedef void* nitattribute_t; /* general C type representing a Nit attribute. */")
		else
			self.header.add_decl("typedef union \{")
			self.header.add_decl("void* val;")
			for c, v in self.box_kinds do
				var t = c.mclass_type

				# `Pointer` reuse the `val` field
				if t.mclass.name == "Pointer" then continue

				self.header.add_decl("{t.ctype_extern} {t.ctypename};")
			end
			self.header.add_decl("\} nitattribute_t; /* general C type representing a Nit attribute. */")
		end
	end