Combine rhubarb, corn starch, and sugar in saucepan. Cook and stir until rhubarb is tender and is thickened. Prepare instant pudding according to package directions. Mix pudding,marshmallows and cool whip together. Pour rhubarb mixture over crust, and let cool to lukewarm. Spread pudding mixture over rhubarb. Chill until firm.
Reserve 1/4 c. of graham cracker mixture to sprinkle on top of pudding