It will be really nice to have line numbers next to the expression block in Pro.
Just to show a need
Sometimes when your code errors out and it says line 17 it's silly to visually count the lines through the naked eye. It can also be cumbersome if you have lots of lines of code. This is where line numbers can be really helpful. A lot of coding programs have it so i think Pro should have this too for:
Calculate Field code block
Expression Builder under Attribute Rules
While writing expressions under symbology and labels
Python Window and etc.