We got Mac leaving the slammer in Van Nuys late Thursday morning, where just hours earlier he had been locked up for allegedly crashing into a power pole and fleeing the scene ... all while drunk.

Our camera guy asks the hard questions. Why did he allegedly bail from the crash site? How much had he had to drink? What did he say to cops to get a glowing suspect review? And is this in any way related to his recent split from Ariana Grande?

Mac was having none of it ... although, we did get a groan out of him. Talk about no comment.

Ariana just tweeted, "Pls take care of yourself."