This gist is some helper functions and a rake task that allows you to quickly, safely , and easily vendor modules into your gems.

1_vendor_modules.rake

require 'fileutils'

def subsume_modules(*vendor_modules)
  options = vendor_modules.pop
  options[:modules] = vendor_modules + Array(options[:additional_modules])
  options[:module_names] = options[:modules].map{|m| m.capitalize.gsub(/_[a-z]/){|s| s[-1..-1].upcase}}
  vendor_modules.each do |vendor_module|
    copy_module vendor_module, options
    subsume_module vendor_module, options
  end
end

def copy_module(vendor_module, options)
  from_path = "#{options[:from]}/#{(options[:locations]||{}).fetch(vendor_module, "lib/#{vendor_module}")}"
  to_path = "#{options[:to]}/#{vendor_module}"
  FileUtils.rm_rf(["#{to_path}.rb", "#{to_path}"])
  globs = ["#{from_path}.rb", "#{from_path}/**/*.rb"]
  globs += Array(options[:additonal_extensions]).map {|ext| "#{from_path}/**/*.#{ext}"}
  vendor_files = FileList.new(*globs)
  vendor_files.each do |vendor_file|
    target_file = vendor_file.gsub(/#{Regexp.escape(from_path)}/,to_path)
    FileUtils.mkdir_p(File.dirname(target_file))
    FileUtils.cp "#{vendor_file}", "#{target_file}"
  end
end

def subsume_module(vendor_module, options)
  vendor_files = FileList.new("#{options[:to]}/#{vendor_module}.rb",
                              "#{options[:to]}/#{vendor_module}/**/*.rb")
  vendor_files.each do |vendor_file|
    subsume(vendor_file, options)
  end
end

def scope_subsumed_constants!(contents, options)
  contents.gsub!(/#{options[:module_names].join('|')}/) {|m| "#{options[:scope]}::#{m}"}
end

def subsume(vendor_file, options)
  contents = File.read(vendor_file)
  unless contents =~ /#{options[:scope]}/
    puts "Processing: #{vendor_file}"
    contents.gsub!(/^(.*\$\:)/,'# \1') if options[:disable_path_munging]
    contents.gsub!(/(require\s+['"])(#{options[:modules].join("|")})/,"\\1#{options[:require_prefix]}/\\2")
    contents.gsub!(/(autoload\s+:[\w:"]+,\s+['"])(#{options[:modules].join("|")})/,"\\1#{options[:require_prefix]}/\\2")
    unscoped = options[:unscoped] && vendor_file =~ options[:unscoped]
    contents = "module #{options[:scope]}\n#{contents}" unless unscoped
    base_name = vendor_file[(options[:from].size+1)..-1]
    unless unscoped
      if ending = options.fetch(:end_placements,{})[base_name]
        before, after = contents.split(ending.first, 2)
        puts after.inspect
        scope_subsumed_constants!(after, options)
        contents = before + ending.last + after
      else
        contents = contents.rstrip + "\nend\n"
      end
    end
    if unscoped
      scope_subsumed_constants!(contents, options)
    end
    contents<<"\n" unless contents[-1..-1] == "\n"
    open(vendor_file, "w") {|f| f.write(contents)}
  end
end

desc "example of how to vendor some modules. Usage: rake vendor[path/to/rails]"
task :vendor, :rails_path do |task, args|
  subsume_modules "active_support","active_model","active_resource",
    :from => args[:rails_path],
    :to => "lib/caring/directory/vendor",
    :require_prefix => "caring/directory/vendor",
    :scope => "Caring::Directory::Vendor",
    :unscoped => /core_ext|whiny_nil/,
    :disable_path_munging => true,
    :locations => {
      "active_support"  => "activesupport/lib/active_support",
      "active_model"    => "activemodel/lib/active_model",
      "active_resource" => "activeresource/lib/active_resource"
    },
    :additonal_extensions => ["yml", "dat"],
    :end_placements => {
       "active_support/json/encoding.rb" => ["end\n\nclass Object", "end\nend\n\nclass Object"] # This file does some core extensions so we have to add the end in the middle of the file.
    }
end

2_example.diff

diff --git a/gems/directory_client/lib/caring/directory/active_model.rb b/gems/directory_client/lib/caring/directory/active_model.rb
index be0f24f..dde14b8 100644
--- a/gems/directory_client/lib/caring/directory/active_model.rb
+++ b/gems/directory_client/lib/caring/directory/active_model.rb
@@ -1,3 +1,4 @@
+module Caring::Directory
 #--
 # Copyright (c) 2004-2010 David Heinemeier Hansson
 #
@@ -22,25 +23,25 @@
 #++
 
 activesupport_path = File.expand_path('../../../activesupport/lib', __FILE__)
-$:.unshift(activesupport_path) if File.directory?(activesupport_path) && !$:.include?(activesupport_path)
-require 'active_support'
-require 'active_model/version'
+# $:.unshift(activesupport_path) if File.directory?(activesupport_path) && !$:.include?(activesupport_path)
+require 'caring/directory/active_support'
+require 'caring/directory/active_model/version'
 
 module ActiveModel
   extend ActiveSupport::Autoload
 
   autoload :AttributeMethods
-  autoload :BlockValidator, 'active_model/validator'
+  autoload :BlockValidator, 'caring/directory/active_model/validator'
   autoload :Callbacks
   autoload :Conversion
   autoload :Dirty
-  autoload :EachValidator, 'active_model/validator'
+  autoload :EachValidator, 'caring/directory/active_model/validator'
   autoload :Errors
   autoload :Lint
   autoload :MassAssignmentSecurity
-  autoload :Name, 'active_model/naming'
+  autoload :Name, 'caring/directory/active_model/naming'
   autoload :Naming
-  autoload :Observer, 'active_model/observing'
+  autoload :Observer, 'caring/directory/active_model/observing'
   autoload :Observing
   autoload :Serialization
   autoload :TestCase
@@ -56,5 +57,6 @@ module ActiveModel
   end
 end
 
-require 'active_support/i18n'
+require 'caring/directory/active_support/i18n'
 I18n.load_path << File.dirname(__FILE__) + '/active_model/locale/en.yml'
+end