I implemented a feature to show line numbers with left margins in diagnostics like GCC-9 does. Ref: https://developers.redhat.com/blog/2019/03/08/usability-improvements-in-gcc-9
I'm in favour of this, but @rsmith warned me a little while ago that we mightn't be able to have stuff like this on by default because scripts probably depend on specific compiler output. Hyrum's Law strikes again :(
(I personally think we should have it on by default and give a way to opt out, rather than opt in.)
I believe even more tools depend on gcc and gcc devs were able to turn it on by default, so kinda bad excuse, especially with opt out flag. Can you check how they did it? Or they just did it with no useless worries?