I have new 12in one piece toilet that I need to fit in a 10in rough in space.

You will most likely not be able to install 12" toilet on 10" rough. Shop for 10" rough-in toilet or relocate the ring away from the wall. However, I have seen once this situation solved by cutting a rectangular niche-like indentation into wall behind the toilet. You may consider this approach, too...

Well, I don't think cutting into the dry wall would leave a very attractive looking installation. Your only option would be to install a 2" offset flange, (see image) or take up the floor and move the closet bend out 2 inches. Good luck and thanks for rating my answer. Tom