In this context, what would be useful is something like a linker-pruning at the source level.
That is, when your code is compiled, the linker can prune code that is never called. Then a feedback mechanism could show which part of the code is actually used (like looking in the .map of the linker).
Google's Closure compiler was doing this for JavaScript, where it matters because network bandwidth is a limited resource in some places. There it was called “tree shaking” if you want the jargon name for it.
That is, when your code is compiled, the linker can prune code that is never called. Then a feedback mechanism could show which part of the code is actually used (like looking in the .map of the linker).
Does something like that exist?