Blend margarine with 2 cups flour and 2 tsp sugar. Pat into 9 x 13 pan and bake for 10 minutes.
Mix rhubarb, 2 cups sugar, cream, egg yolks, 4 Tbsp flour and salt. Pour over crust and bake for 45 minutes.
Beat egg whites into peaks. Add 3/4 cup sugar, 2 Tbsp at a time. Add vanilla. Put on rhubarb mixture and top with coconut if desired. Bake until light brown.