If the damage that would be dealt by Rile is prevented, the creature still gains trample until end of turn.

If Rile targets a creature with 1 toughness, that creature won't be destroyed until after you've drawn a card. Its abilities may affect that draw or trigger on that draw if appropriate.

If the target creature is an illegal target by the time Rile resolves, the entire spell doesn't resolve. You won't draw a card.