Combine sugar and flour; add rhubarb and pineapple, tossing lightly to coat. Pour fruit mixture into unbaked pie shell.

Combine flour and sugar. Cut in butter until crumbly. Sprinkle topping mixture over filling. Bake at 425 degrees for 15 minutes. Reduce oven temperature to 400 degrees and bake 25-30 minutes more or until filling is hot and bubbly. Cool.