How about vinegar????????About the only thing that will damage glazed porcelain is undue scrubbing with an abrasive over a long period of time, OR products containing hydrofluoric acid or ammonium biflouride.
You will NOT find those two chemicals available for home use, as they are extremely dangerous. I used them in a car wash for wall cleaning, and they are also sometimes used in products called aluminum brightener or sometimes, a wheel cleaner for cars. How dangerous are they? They will go through the skin without you feeling it, and start to eat your bones. No &^%. That's why when mixed for commercial use they often contain sulfuric and or hydrochloric acids--so you can feel the burn on your skin, and if you are smart enough, do something about it before real damage occurs. ! Fun, eh?
Want the truth on the "acid" in Coke? Do the Acids in Coca-Cola Make It Harmful to Drink?
Now, if you want to clean the stains in your toilet? Use toilet bowl cleaner. There are a million of those products out there. Are your stains rust based? Use "Iron-Out" or something similar.