Preheat oven to 375 degree F. For filling, in a large bowl stir together sugar, flour, and cinnamon; stir in undrained rhubarb.
Line a 9-inch pie plate with pastry; trim. Transfer filling to pastry. Trim bottom pastry to edge of plate. Cut slits in remaining pastry; place on filling and seal. Crimp edge as desired. If desired, brush with milk; sprinkle with sugar.
Cover edge of plate with foil. Bake for 25 minutes; remove foil. Bake for 20 to 30 minutes more or until filling is bubbly in center and pastry is golden. Cool on a wire rack. Makes 8 servings.