Page 496 of Collins Cobuild English Usage reads
> You form 'yes/no'-questions with **used to** by putting _did_ in front of the subject, followed by **use to**.
>
> Did you use to do that, when you were a kid?
>
> **WARNING**: many people use *used to* instead of *use to* in
> questions. However, some people consider that this use is incorrect. 
>
>*Did you **used to** play with your trains?*
> 
> If the 'wh'-word is the object of the clause, or part of the object, you
> use the auxiliary *do* after it, followed by the subject and *used to*.
>
> *What did you **used to** do on Sundays?*

Why isn't **use to** possible in the second structure?