Rectangular Marquee Measurement Tool
A tool that allowed me to make rectanular measurements would be useful. It could behave like the selecting marquee tools in PS.
This would be valuable because it would quickly give more specs to parts of the mockup that are not always XD elements. For example, if there was part of a mockup that was a screenshot, the XD measurement tools are not as accurate. Also is helpful for finding sizes of invisible containers. This would allow developers to freely grab measurements quickly for anything on the page.
I agree. It is also very useful for developers. For example, imagine that you have 2 lines of text. You can see the distance by holding the ALT key, however it does not take into consideration the line-height of the text. The actual gap between the two elements (texts) is much bigger.
Nick Gooding commented
Additionally, once a measurement is started holding Shift (or something else) could magnet anchor to other element edges.
The dimensions could display the same way as the PS tool.