ruby@2.0: vendor gem like we do pip for Python

Formula/ruby@2.0.rb

@@ -3,7 +3,7 @@ class RubyAT20 < Formula
   homepage "https://www.ruby-lang.org/"
   url "https://cache.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p648.tar.bz2"
   sha256 "087ad4dec748cfe665c856dbfbabdee5520268e94bb81a1d8565d76c3cc62166"
-  revision 1
+  revision 2
 
   bottle do
     sha256 "f7337fa641d1e7a345d2ae693bc98ff47e2d109321d0d9224007a9fed827acd6" => :high_sierra
@@ -26,6 +26,26 @@ class RubyAT20 < Formula
   depends_on "openssl"
   depends_on :x11 if build.with? "tcltk"
 
+  # This should be kept in sync with the main Ruby formula
+  # but a revision bump should not be forced every update
+  # unless there are security fixes in that Rubygems release.
+  resource "rubygems" do
+    url "https://rubygems.org/rubygems/rubygems-2.6.14.tgz"
+    sha256 "406a45d258707f52241843e9c7902bbdcf00e7edc3e88cdb79c46659b47851ec"
+  end
+
+  def program_suffix
+    build.with?("suffix") ? "20" : ""
+  end
+
+  def ruby
+    "#{bin}/ruby#{program_suffix}"
+  end
+
+  def api_version
+    "2.0.0"
+  end
+
   def install
     args = %W[
       --prefix=#{prefix}
@@ -67,17 +87,37 @@ class RubyAT20 < Formula
 
     system "make"
     system "make", "install"
+
+    # This is easier than trying to keep both current & versioned Ruby
+    # formulae repeatedly updated with Rubygem patches.
+    resource("rubygems").stage do
+      ENV.prepend_path "PATH", bin
+
+      system ruby, "setup.rb", "--prefix=#{buildpath}/vendor_gem"
+      rg_in = lib/"ruby/#{api_version}"
+
+      # Remove bundled Rubygem version.
+      rm_rf rg_in/"rubygems"
+      rm_f rg_in/"rubygems.rb"
+      rm_f rg_in/"ubygems.rb"
+      rm_f bin/"gem#{program_suffix}"
+
+      # Drop in the new version.
+      (rg_in/"rubygems").install Dir[buildpath/"vendor_gem/lib/rubygems/*"]
+      rg_in.install buildpath/"vendor_gem/lib/rubygems.rb"
+      rg_in.install buildpath/"vendor_gem/lib/ubygems.rb"
+      bin.install buildpath/"vendor_gem/bin/gem" => "gem#{program_suffix}"
+    end
   end
 
   def post_install
     # Customize rubygems to look/install in the global gem directory
     # instead of in the Cellar, making gems last across reinstalls
-    config_file = lib/"ruby/#{abi_version}/rubygems/defaults/operating_system.rb"
+    config_file = lib/"ruby/#{api_version}/rubygems/defaults/operating_system.rb"
     config_file.unlink if config_file.exist?
     config_file.write rubygems_config
 
     # Create the sitedir and vendordir that were skipped during install
-    ruby="#{bin}/ruby#{program_suffix}"
     %w[sitearchdir vendorarchdir].each do |dir|
       mkdir_p `#{ruby} -rrbconfig -e 'print RbConfig::CONFIG["#{dir}"]'`
     end
@@ -86,16 +126,8 @@ class RubyAT20 < Formula
     mkdir_p rubygems_bindir
   end
 
-  def abi_version
-    "2.0.0"
-  end
-
-  def program_suffix
-    build.with?("suffix") ? "20" : ""
-  end
-
   def rubygems_bindir
-    "#{HOMEBREW_PREFIX}/lib/ruby/gems/#{abi_version}/bin"
+    "#{HOMEBREW_PREFIX}/lib/ruby/gems/#{api_version}/bin"
   end
 
   def rubygems_config; <<~EOS
@@ -113,7 +145,7 @@ class RubyAT20 < Formula
           "lib",
           "ruby",
           "gems",
-          "#{abi_version}"
+          "#{api_version}"
         ]
 
         @default_dir ||= File.join(*path)