David Letterman -- Un-Retires to Roast Donald Trump (VIDEO)

David Letterman must have been sitting at home licking his chops over all the material Donald Trump was providing -- so, he came out of retirement!

Well, for one night anyway ... Letterman joined Steve Martin and Martin Short on stage during their show in San Antonio last night -- and he came armed with a timely Top 10: "Interesting facts about Donald Trump."

Y'know how #2 was frequently better than #1 with Dave's Top 10s? Not this time ... make sure you watch to the end.