class Bundler::Source::Rubygems

def cache(spec, custom_path = nil)
  if builtin_gem?(spec)
    cached_path = cached_built_in_gem(spec)
  else
    cached_path = cached_gem(spec)
  end
  raise GemNotFound, "Missing gem file '#{spec.full_name}.gem'." unless cached_path
  return if File.dirname(cached_path) == Bundler.app_cache.to_s
  Bundler.ui.info "  * #{File.basename(cached_path)}"
  FileUtils.cp(cached_path, Bundler.app_cache(custom_path))
end