As a followup to this question: Can I safely connect two GPIO pins to each other (ie. for testing purposes)? And can I safely connect an input pin to either +VDD (3.3 V) or to Ground? Or do I need a resistor each time? I would expect that the maximum input current for the pins is equal to the maximum output current, so that this shouldn't damage anything.
Note: I know that connecting the 5V output to anything could fry the Pi. This question only is about Pins running with 3.3V.