"When you are up to your neck in shit, all you can do is sing."
I just checked some credible online dictionaries like Longman, Macmillan and American Heritage and they all had "sing" as a verb only. Though there was an entry as a noun in the latter, obviously it wasn't relevant to the context here.
On the other hand, I checked and saw the infinitive form of "sing" as "to sing" so I'm confused and very curious to find the grammatical point behind this usage.
Plus I'd like to know how it would be if we replaced "sing" with "struggle" here. Can both "noun" and "verb" forms of a given verb be used here after "is"?