The normal, completely unremarkable negative (with Do-Support) of the idiom used to in
- I used to like marshmallows.
/a'yustuˌlayk'marʃˌmɛloz/
is pronounced (note the /st/ in both forms -- no /zd/ for used in this idiom)
- /aydɪdṇ'yustuˌlayk'marʃˌmɛloz/
and that's not a problem. In speech.
There isn't even much confusion with the almost identical idiom be used to, meaning 'be accustomed to' -- most English speakers don't even notice that there are two of them.
But the past-tense-with-presupposition idiom used to does pose a problem, in writing. In speech it's treated as an infinitive, as required by Do-Support, no problem. But in writing used doesn't look like an infinitive; in fact, it's a past participle. But the idiom is spelled used to. Hence, there are two choices:
a) I didn't used to like marshmallows.
b) I didn't use to like marshmallows.
Both of these look wrong to experienced English readers, because both of them contain sequences readers have been taught to avoid as products of grammatical mistakes:
(a) contains didn't used, which is wrong -- Do-Support doesn't make a participle used.
So that looks wrong.(b) contains use to instead of used to, which is the idiom, so use gets a /z/ .
And that looks wrong, too.
Too bad, so sad, English orthography flunks again.