Photoshop how to convert shape (rectangle) to bitmap

I am coming from old Adobe Fireworks, where converting any shape into a bitmap is as simple as selecting it and click menu Modify -> Flatten Selection, and that’s it.

Unfortunately I have found out there is nothing like that in Photoshop (well, at least nothing I would know of).

Can anyone explain to me how to do this simple task in Photoshop (Google search did not help with this at all)?