How to calculate a rotated a Rect struct (not a RectTransform)?

I'm working on a game that uses a grid system, and I'm trying to figure out how to update the data model when the player rotates one of their assets. Since it's a grid, I'm only worried about rotations in increments of 90° at a time.

For example, if you rotated a horizontal 3-tile block that covers {3,4} {4,4} {5,4} by 90°, it would end up as a vertical line covering {4,5} {4,4} {4,3}.

Is there a method or class for this kind of thing? How can I calculate the positions covered by a Rect after it has been rotated?