Pixel to pixel distance measurement tool
A tool to click at any point in the artboard and measure the distance from that point to another point anywhere else in the same artboard.
Closing as per the most recent comment.
The alt key will show the distance between two different elements. The tool I had in mind was one that shows distance between *any* two selected points in the artboard. Not necessarily between two elements.
But then I really can't remember the use case I had for this. ╮(. ❛ ᴗ ❛.)╭
The alt key tool will suffice for now.