I have a chapter in my book (The Encyclopedia of Men's Clothes) on stain removal and here's what I have on rust:
Rust stains cannot be removed in normal laundering. Do not use chlorine bleach, it will make the stains permanent.
Small stains may be removed with a few drops of a commercial rust remover, or by repeated applications of lemon juice and salt on the stain. Do not let the garment dry between applications. Rinse thoroughly and launder with a liquid laundry detergent and oxygen bleach, following directions.
If safe for fabric, boil in solution of 4 teaspoons of cream of tartar per pint of water. Rinse thoroughly.
Severe rust stains may be removed with a commercial rust remover available at grocery or hardware stores. Follow package instructions. Caution: rust removers that contain hydrofluoric acid are extremely toxic, can burn the skin, and will damage the porcelain finish on appliances and sinks.