You're mis-understanding what "you don't pay for what you don't use" means. It means that a program that does not contain [] in it does not include code for []. There's no "mandatory runtime" code for []. It doesn't mean you can use [] in some special way and somehow get less code.
And even if we were talking about your understanding, I don't see why get_unchecked would be disqualified. unsafe is a part of the language for a reason.
And even if we were talking about your understanding, I don't see why get_unchecked would be disqualified. unsafe is a part of the language for a reason.