Preheat the oven to 400 degrees F.
In a large bowl, combine the rhubarb, cranberries, sugar, and cornstarch. Mix everything together well.
Place one pie crust in a pie plate. Pour the cranberry rhubarb mixture inside. Dollop small pieces of the butter across the top of the filling.
Create the lattice crust for the top of the pie. Todo so, cut the other pie crust into 10 to 12 strips, each about ¾-inch wide(and the length of the rolled-out crust). Lay half the strips across the pie horizontally. Weave the remaining strips vertically through, in an over/under pattern. Use a fork to press the strips down into the bottom crust along the edges of the pie plate.
Place aluminum foil over the pie, with a circle cutout of the center so that only the edges of the crust are covered. Bake at 400degrees for 20 minutes covered, then remove the foil and bake for another 20minutes (for about 40 minutes of total cooking time), until the pie crust is golden brown and cooked through.
Let the pie cool completely before eating. Enjoy!