Spankings?

Kayla • 26. Wife. Mom. Type 1 diabetic.
I realize that we still have quite a while until this may happen but I am still so mad. My mother in law is going to be watching our son after I go back to work. My husband came home yesterday & said that when his mom watches our son that she has no problem spanking him if he is bad. Both my husband & I had spankings when we were younger but I that is not how I want to raise my child. I am not sure how my husband feels about the situation but I absolutely do not like the thought of spanking my child & I definitely do not like the thought of anyone other than my husband or I doing it. Maybe I am being crazy but it is not my MIL's place to punish my child in that way. How should I go about talking to her about this? I'm afraid she is the kind of parent that will not listen to our rules & do what she wants behind our backs.