MICROWAVE marshmallows and reserved pineapple liquid in large microwaveable bowl on HIGH 2 to 3 min. or until marshmallows are completely melted and mixture is well blended, stirring after each minute.
REFRIGERATE 30 min. or until slightly thickened; stir until blended. Gently stir in pineapple and COOL WHIP; pour into crust.
REFRIGERATE 3 hours or until firm.