That is definitely one argument (and I do see where you're coming from -- especially if your error path has a defer that does cleanup or something complicated l

That is definitely one argument (and I do see where you're coming from -- especially if your error path has a defer that does cleanup or something complicated like that). However, I don't think spending a significant amount of time mocking out every struct method that has an error return is a worthwhile investment -- the majority of 'if err != nil { return err }' cases are not going to be interesting and the ones that are usually don't even need mocking to test because they are significant enough error cases that they are easy to trigger using the real version of whatever struct. And of course you should test error paths to make sure that your code does validate things.