docs: def path_in_cache(self, filename, metahash):
          """Generates the path to a file in the mh cache.
  
          The generated path does not imply the file's existence!
  
          Args:
            filename: Filename relative to buildroot
            rule: A targets.SomeBuildRule object
            metahash: hash object
          """
          cpath = self._genpath(filename, metahash)
          if os.path.exists(cpath):
              return cpath
          else:
              raise CacheMiss