When Promising Future is played and the opponent chooses Snapvine while the other player chooses Riptide Rex, can Snapvine's ETB effect trade with Rex, and can Rex kill Snapvine before its effect resolves?

Ruling: Both units enter play and their ETB triggers go on the chain, with the last trigger (Rex) resolving first. Rex can kill Snapvine before Snapvine's trigger resolves, but Snapvine's trigger will still resolve even if Snapvine is dead.

Sequence:

• Both players choose cards for Promising Future (opponent chooses first, then you)
• All chosen cards become pending items
• Your card (Snapvine) goes from pending onto the chain first and resolves immediately as a permanent
• Snapvine's play trigger is added as a pending item
• Opponent's card (Rex) goes onto the chain next and resolves immediately as a permanent
• Rex's trigger is added as a pending item
• Snapvine's trigger is added to the chain and targets are chosen
• Rex's trigger is added to the chain and targets are chosen
• Rex's trigger resolves first, then Snapvine's trigger resolves

Nuances:

• If Rex targets Snapvine, Snapvine will be dead but its trigger still resolves and can kill Rex
• If Rex targets something else, both triggers resolve and the units trade