```ruby
require File.expand_path('../gir_ffi_test_helper.rb', File.dirname(__FILE__))
require "gir_ffi/callback_helper"

describe GirFFI::CallbackHelper do
  describe ".map_single_callback_arg" do
    it "correctly maps a :struct type" do
      GirFFI.setup :GObject

      cl = GObject::Closure.new_simple GObject::Closure::Struct.size, nil

      cinfo = GObjectIntrospection::IRepository.default.find_by_name 'GObject', 'ClosureMarshal'
      ainfo = cinfo.args[0]

      r = GirFFI::CallbackHelper.map_single_callback_arg cl.to_ptr, ainfo

      assert_instance_of GObject::Closure, r
      assert_equal r.to_ptr, cl.to_ptr
    end
  end
end
```

Test callback helper without setting up GObject.
user
```ruby
require File.expand_path('../gir_ffi_test_helper.rb', File.dirname(__FILE__))
require "gir_ffi/callback_helper"

describe GirFFI::CallbackHelper do
  describe "::map_single_callback_arg" do
    it "maps a :struct type by building the type and wrapping the argument in it" do
      cinfo = get_introspection_data 'GObject', 'ClosureMarshal'
      ainfo = cinfo.args[0]
      ifinfo = ainfo.argument_type.interface

      assert_equal :struct, ifinfo.info_type

      struct_class = Class.new
      mock(GirFFI::Builder).build_class(ifinfo) { struct_class }
      mock(struct_class).wrap("dummy") { "good-result" }

      r = GirFFI::CallbackHelper.map_single_callback_arg "dummy", ainfo

      assert_equal "good-result", r
    end
  end
end
```
assistant