Blend cornstarch, salt, and sugar. Add to fruit and mix. Place filling in deep 8-inch pie pan, heaping center of filling slightly above top of pan so that it will support pastry. Dot with butter. Moisten rim of pan. Roll pastry, cut several gashes in it, and place over filling. Trim edge – leaving ½-inch margin beyond edge of pan. Turn margin over and flute edge of crust, pressing firmly to pan. Sprinkle lightly with sugar. Bake in hot oven (425 degrees F) until crust is browned and rhubarb tender (about 30 minutes).