rusty charcoal chimney starter, safe to use?
Yes, it's perfectly safe to use basically until it falls apart. It gets very hot in use, which prevents any sort of rust-proof coating from sticking, but the rust is harmless. You could try oiling it after use if you wanted to reduce further rust - that oil will burn off every time it's used, but will help to slow rusting when not in use - however, it's not really needed (could be regarded as a waste of oil.)