Add cranberries and sugar to the bowl of a food processor. Pulse until the cranberries are finely chopped small, with some coarse pieces remaining. Transfer to a large bowl. Cover and refrigerate overnight (or at least 2 hours).
Stir in the crushed pineapple, nuts and whipped topping until thoroughly combined.
Fold in marshmallows.
Cover and refrigerate for 2 hours before serving.
Notes
Store Cranberry Fluff in the refrigerator for up to 3 days.