If you look at the size of most Cornsnakes, and the size of rats, common sense tells you that rats are too big of prey for Cornsnakes.
I have had friends who have had snakes die way before their time, and a necropsy shows large fatty deposits, and the animal was fed rats.
I have talked with people in the Carpet Python (snakes that actually are large enough to handle rats) world who have even looked into starting to offer a variety of lower fat prey (chicks, quail) because they have lost Carpet pythons from being fed too fatty of a diet with rats.
You have to do what you feel is best for your animal(s).
I have fed weanling rats to females recovering from egg laying, but I've had much more success giving reptile vitamin supplement (injected into f/t mice) to malnourished snakes or females having a hard time recovering from egg laying. I do not give it to otherwise healthy snakes.
Proof is in the pudding, as they say.