In all practicality it will function like a “Line Tool”/“Curve Tool” It could simply display the length in the bottom left like with the “Marquee Tool” and if possible leave a temporary mark at the start and end of the rulers line (Unnecessary but it would help). The reason this would shine over other methods is most commonly in cases where the user would need to measure distances that were not in a straight line like curves or outlines of sprites (I use the “Curve Tool” ALL the time). In summery I believe this would boost not just mine but others workflow greatly as well in theory be easy to implement due to most of the functions already existing in Aseprite.