There is an attachment. I've used it. I like it. Oddly, I don't find it on any of the usual suspect web sites.
It works on the opposite side of the rail from the circular saw, so don't let that confuse you when you try to set it up. It's a rectangular piece of plastic with a rectangular cut out, you snap the saw into that cut-out,l put the recess in the other part of the plastic over the rail, make your cut.