The phrase "trust to luck" means to rely mainly on chance or luck for a favorable outcome. Here are some synonyms for this phrase that can be used to express similar meaning: gamble, take one's chances, leave it to fate, leave it to chance, roll the dice, wing it, go with the flow, cross one's fingers, hope for the best, play by ear, let things happen, take potluck, throw caution to the wind. All these synonyms indicate that one is taking a risk and is uncertain about what the outcome will be. When you trust to luck, you may sometimes win big, but more often than not, you'll end up disappointed.