Learn English – ‘He may need not to …’ or ‘He may not need to …’

I suppose will is very slightly more accurate, but I would say that either word is correct in everyday usage. The word 'will' connotes a more definite occurrence; if we say "that will happen", we are indicating that there is no doubt about it, where when we say "that may happen", we are specifically saying it may or it may not happen. In your sentence, there isn't any certainty about whether the passport will be handy, though there are situations where it will be required (that you may not foresee) and others where it is merely convenient.

But either word is correct in the sentence, and there is no real difference in their meaning here.

You need the permission of the compiler for things to be accepted, and the compiler is denying you that permission. "May I do this?" "You may not."

May is just as strong as must. It's only weak in the positive sense, where once you're granted permission, you're not obligated to do it.