Never had a lager go that fast, but I always ferment on the low end of temperature. I guess if you pitched a large quantity of healthy yeast and fermented on the high end, it's definitely possible. If it is fermented out, your D-rest will be less effective. But diacetyl can be eaten up during lagering too, so I'd just make sure you lager it for a good long time (depending on OG, maybe 8-12 weeks). Then taste it to see if there's still any diacetyl. If you're bottling, you'll want to add some dry lager yeast (~1/2 pack per 5 gallons) to the bottling bucket just to be safe because of the long lagering.