Discarding for a Cost

Cards that required you to Discard for a Cost usually have Discard x card(s) or Discard x card(s) to the Graveyard at the beginning of the card lore.

You must discard to pay the cost, you cannot activate the card if you cannot pay the cost, for example, if you do not have any cards in your hand. If you have a card that has Discard x card(s) to the Graveyard as the cost, and "Macro Cosmos" is face-up and active, you cannot pay the cost, since you are fully aware that the card will not be sent to the Graveyard.