BRUNO MARS GOT RATTED OUT BY THE BATHROOM ATTENDENT AT THE HARD ROCK HOTEL!

Never underestimate the humble bathroom attendant -- because according to Las Vegas police ... it was an Altoid-slingin', paper towel passer who helped cops bust Bruno Mars in a casino bathroom this weekend.
TMZ has obtained the police report from the incident, in which LVMPD officials state a bathroom attendant at the Hard Rock Hotel informed security "there was a man in the bathroom stall with possibly a man using drugs."

According to the report, the attendant went on to say he could "see a male in a striped red shirt with a baggy of white powder substance."

Once law enforcement officers made their move -- cops say they asked Bruno to "give over whatever narcotics he had." According to the report, Bruno then "pulled out a white powder substance, which was consistent with cocaine, from his left-front jeans pocket."

Cops say Bruno --aka Peter Hernandez-- "admitted he did a foolish thing and has never used drugs before."

The report shows ... once the substance "came back positive for cocaine," Bruno was shipped off to the Clark County Detention Center and booked for possession of narcotics.

Always remember to tip your bathroom attendant.