code: def get_dependencies_from_cache(ireq):
      
      if ireq.editable or not is_pinned_requirement(ireq):
          return
      if ireq not in DEPENDENCY_CACHE:
          return
      cached = set(DEPENDENCY_CACHE[ireq])
  
      # Preserving sanity: Run through the cache and make sure every entry if
      # valid. If this fails, something is wrong with the cache. Drop it.
      try:
          broken = False
          for line in cached:
              dep_ireq = pip_shims.shims.InstallRequirement.from_line(line)
              name = canonicalize_name(dep_ireq.name)
              if _marker_contains_extra(dep_ireq):
                  broken = True   # The "extra =" marker breaks everything.
              elif name == canonicalize_name(ireq.name):
                  broken = True   # A package cannot depend on itself.
              if broken:
                  break
      except Exception:
          broken = True
  
      if broken:
          del DEPENDENCY_CACHE[ireq]
          return
  
      return cached
docs: Retrieves dependencies for the given install requirement from the dependency cache.
  
      :param ireq: A single InstallRequirement
      :type ireq: :class:`~pip._internal.req.req_install.InstallRequirement`
      :return: A set of dependency lines for generating new InstallRequirements.
      :rtype: set(str) or None