Add build tag conditon when build tag isn't passed

Author: sue445

_tools/ruby_h_to_go/lib/ruby_h_to_go/go_util.rb

@@ -16,16 +16,35 @@ def self.snake_to_camel(str)
     def self.generate_initial_go_file(go_file_path)
       return if File.exist?(go_file_path)
 
-      ruby_build_tag = GoGem::Util.ruby_minor_version_build_tag
-
-      File.binwrite(go_file_path, <<~GO)
+      header = +<<~GO
         // THE AUTOGENERATED LICENSE. ALL THE RIGHTS ARE RESERVED BY ROBOTS.
 
         // WARNING: This file has automatically been generated
         // Code generated by ruby_h_to_go. DO NOT EDIT.
 
-        //go:build #{ruby_build_tag}
+      GO
+
+      ruby_build_tag = GoGem::Util.ruby_minor_version_build_tag
+
+      header <<
+        if ruby_build_tag == RubyHToGo.config.default_tag
+          other_tags = RubyHToGo.config.available_tags - [RubyHToGo.config.default_tag]
+          condition = other_tags.map { |tag| "!#{tag}" }.join(" && ")
 
+          <<~GO
+            // FIXME: https://pkg.go.dev/ doesn't support custom build tag.
+            //        Therefore, if no build tag is passed, treat it as the default tag
+            //go:build #{ruby_build_tag} || (#{condition})
+
+          GO
+        else
+          <<~GO
+            //go:build #{ruby_build_tag}
+
+          GO
+        end
+
+      header << <<~GO
         package ruby
 
         /*
@@ -38,6 +57,8 @@ def self.generate_initial_go_file(go_file_path)
         )
 
       GO
+
+      File.binwrite(go_file_path, header)
     end
 
     C_TYPE_TO_GO_TYPE = {