The normal, completely unremarkable negative (with _Do_-Support) of the idiom [*used to*][1] 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 produce a participle *used*.  
So **that** looks wrong.  
(b) contains _**use to**_ instead of _used to_, which is the idiom, so _use_ is pronounced with a /z/.  
And **that** looks wrong, too.

Too bad, so sad, English orthography flunks again.


  [1]: https://english.stackexchange.com/a/72205/15299