rigidbody2D axis constraints

I have read some posts here in the forum, and I found out that the new Unity 2D tools doesn't support constrainst for the x-axis, and even the y-axis. I'm not entirely sure on how I would go on coding it instead, or if it's even possible. I hope you could share some ideas about this. It would be helpful for a project I'm working on. And I wish Unity would add it soon. Thank you so much!