This isn't really a very useful solution. This change may bring down compile times but compile times are still atrociously slow. You change one line of one final class and it takes Xcode over a minute to recompile. That's criminal.

The rule you are breaking is that capitalization matters when you spell a name. You're saying xFile = ... So the Swift compiler looks for the term xFile declared as a variable name in scope, to which something can be assigned. But the word xFile doesn't appear as a variable name in scope — just as the compiler is telling you. What does appear as a variable name in scope is xfile, which as far as Swift is concerned is a totally different name.

Bump. I repeat the question. Xcode 13 is final. I need the Vary For Traits feature. Some of my apps use it. Where is it?

The way to see all environment variable values is to add a run script to your build phases.