Return path to rubygems.rb
Whether a simpler solution exists depends on what your motivation is.

Let's look inside the implementation of Gem::Commands::WhichCommand#find_paths.

  def find_paths(package_name, dirs)
    result = []

    dirs.each do |dir|
      Gem.suffixes.each do |ext|
        full_path = File.join dir, "#{package_name}#{ext}"
        if File.exist? full_path and not File.directory? full_path then
          result &lt;&lt; full_path
          return result unless options[:show_all]
        end
      end
    end

    result
  end


That's the code you need.  If there were a simpler way, find_paths would have used it.

However, there is a way to cheat: Kernel#require needs to do a very similar thing when it actually tries to load a module:


  require(name) → true or false
  
  Loads the given name, returning true if successful and false if
  the feature is already loaded.
  
  If the filename does not resolve to an absolute path, it will be
  searched for in the directories listed in $LOAD_PATH ($:).
  
  If the filename has the extension “.rb”, it is loaded as a source
  file; if the extension is “.so”, “.o”, or “.dll”, or the default
  shared library extension on the current platform, Ruby loads the
  shared library as a Ruby extension. Otherwise, Ruby tries adding
  “.rb”, “.so”, and so on to the name until found. If the file named
  cannot be found, a LoadError will be raised.
  
  For Ruby extensions the filename given may use any shared library
  extension. For example, on Linux the socket extension is “socket.so”
  and require 'socket.dll' will load the socket extension.
  
  The absolute path of the loaded file is added to $LOADED_FEATURES
  ($"). A file will not be loaded again if its path already appears in
  $". For example, require 'a'; require './a' will not load a.rb
  again.


Or, it might be more accurate to say that find_paths simulates what require would do.

Therefore, if you are willing to let Ruby actually load the file first, then you could ask Ruby after the fact:

require 'rubygems'
$LOADED_FEATURES.grep /\/rubygems\./