Going to McGill for college is going to a place where administration actually treats you like the adult, which you are. By Montréal law, your leased apartment, which includes your dorm room, is your private property, and is only subject to search with proper warrant. This means that the administration does not have the authority to do any sort of dorm check for drugs or alcohol. In fact, it is not even against campus policy to have alcohol in the dorms, as it is assumed that everyone is of age. McGill has school-sponsored drinking for a variety of activities, including freshman orientation. Drinking in Montréal just isn't an issue if you're over 18, in the city or on the campus.
What McGill is likely to get involved in is campus theft, vandalism, disturbing campus peace, smoking in unauthorized areas, or blatant drug use on school property. However, unlike many schools in the U.S., if you are charged with possession, you will not just be charged by campus security and suffer an academic punishment. At McGill, they treat you like an adult, and that includes adult penalties. In general, McGill administration does not care what students do, so long as it does not interrupt the academic environment for the rest of the student body.