Is it safe to connect two MCU pins directly to each other?
It's not a problem but if you are worried that there maybe a conflict of outputs then connect them with a 1k ohm resistor. This will limit the current accordingly.

If however, the input needs really fast edges then when you are sure it is working, bridge the 1k ohm with a short or 100 ohms maybe.

The added bonus of having the resistor is that if you have the design commited to PCB then it gives you an easy re-wire option.

It's the same problem with unused inputs - tie them to ground or Vcc and you risk heavy current draw from the device without any easy way to work out what is going on - tie via a 1k if unused or maybe 10k.

Overload limiting is done on some chips but only by virtue that the transistors cannot inherently supply "amps" but this can work against you because if you have several o/p pins shorted to ground (or each other) and each one is limiting gracefully, you may still exceed the max rating of current into the power pins of the device.

For the MSP430 series 2 page 21 of this document says: -



This tells me that if the combined output currents of several pins hits a certain limit then the output capabilities of any other pins (even those that may be lightly loaded) may be impaired by the same amount or possibly worse.