I won't tell if you don't
A lazy way of saying you're not going to keep someone's secret, but you're going to act like you are. It's like a cheap promise that costs nothing and gives everything away.
'I won't tell if you don't,' he said, and then told the whole school.
'I won't tell if you don't,' she said, and then texted her crush.
He said, 'I won't tell if you don't,' and then he posted it on Instagram.