Mock go function that modifies and returns the value passed as argument

Question

I have a function in Go that takes an input argument, modifies it, and finally returns it. To be concrete, this function persists some data in DB using the Bun ORM and returns the persisted entity. That said, I have a unit test where I mock this function, and would need the mocked function to return the variable it receives as an argument. Let's better explain it with a fairly simple example:

package doer

type Something struct {
    Message string
}

type Doer interface {
    DoSomething(s *Something) *Something
}

func TestDoer(t *testing.T) {
    var response *doer.Something

    request := &doer.Something{
        Message: "foo",
    }

    sut := mocks.Doer{}
    sut.On("DoSomething", mock.Anything).Run(func(args mock.Arguments) {
        response = args.Get(0).(*doer.Something)
    }).Return(response)

    r := sut.DoSomething(request)

    assert.Equal(t, request.Message, r.Message)
}

The issue I'm facing here is that the assertion triggers a panic because r is nil. After thinking about it, I think this happens because Return is executed before the function passed to Run and at this point response is nil, so it always returns nil. That said, I would like to know if there is any way of achieving what I want. Please take into account that I need to honor the interface, I already know that I could just rely on the pointer passed to the function as an input argument and get rid of the return value, but I need to stick to this contract.

I'm using testify as the mocking framework