Matthew McConaughey Brings Back the Mustache for New Undercover Informant Movie

Matthew McConaughey is going undercover for his new movie ... and his new mustache says it all.

Matt was seen Friday on the set of his new movie, "White Boy Rick," in which he plays an undercover informant for cops who goes rogue. The guy ends up getting busted for drug-trafficking and sentenced to life in prison.

The flick takes place during the 1980s ... which is clear by his new facial hair. It's also dropping early next year. We've been informed.