You disappoint me pjmlp, you're not a true graybeard unless you use Vim or Emacs :o)
The slightly more serious question: is there anything like this in Vim or Emacs-land? I somehow doubt it... I, for one, have never seen something like it outside of the big IDEs (Visual Studio, IntelliJ, etc.)
For example on Visual Studio, paths not taken on conditional code get grayed out.