Sometimes it’s so much easier to draw a line, using a familiar click-drag-release method, instead of click-click-click method Dimension Tool uses to Manual Dimensions!

I really with to be able to covert a viable line (an open straight line with no handles and two points only, being it a live shape or not) into a dimension, with zero offset and no extension lines.

It IS possible to do today, but using some shenanigans I show in a GIF attached. The way extensions can’t be set to zero is a problem though, and should be reported separately.

Some plugins and scripts allow this exact function, I don’t see why Ai can’t have it.