how to move rectangle and make one side of it shorten during the animation

I have two rectangles. One has a stroke and one is solid. i'm trying to get the solid to move to the right but shorten the left side of the rectangle as it moves. Not sure how complicated that is?? Been working on it for a few hours and can't figure it out. any help would be great. thank you.

Animation at the 8-12second range

i figured it out using image mask. thanks