Hardware Interrupts

I was wondering if an interrupt pin number can be used in its interrupt function for example:

const int buttonInt = 0; // buttonInt is a momentary push button connected to pin 2 on the Uno

attachInterrupt(buttonInt, upward, LOW);

void loop()
{

if (digitalRead (buttonInt) == LOW)
{
digitalWrite(forwards, HIGH);
}

}

if digitalRead (buttonInt) == LOW)

That will read pin 0, not pin 2. So, no.

const uint8_t buttonPin = 2; // a momentary push button connected to pin 2 on the Uno
void setup() {
    pinMode(buttonPin, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(buttonPin), upward, LOW);
}
void loop() {
    if (digitalRead (buttonInt) == LOW) {  // This is completely unnecessary, because you already check the LOW button state using an interrupt.
        digitalWrite(forwards, HIGH);
    }
}

attachInterrupt(buttonInt, upward, LOW);It is unusual to use LOW as an ISR trigger state because the ISR will be triggered constantly while the pin is LOW. It would be more usual to use FALLING to trigger the ISR when the pin goes LOW, but you have not said what you are trying to do or why.