Remove sitedir from LOAD_PATH

Library/Homebrew/brew.sh

@@ -724,7 +724,7 @@ then
 fi
 
 # Disable Ruby options we don't need.
-RUBY_DISABLE_OPTIONS="--disable=rubyopt"
+RUBY_DISABLE_OPTIONS="--disable=gems,rubyopt"
 
 if [[ -z "${HOMEBREW_RUBY_WARNINGS}" ]]
 then

Library/Homebrew/standalone/load_path.rb

@@ -1,8 +1,12 @@
 # typed: true
 # frozen_string_literal: true
 
-require "pathname"
+# We trust base Ruby to provide what we need.
+# Don't look into the user-installed sitedir, which may contain older versions of RubyGems.
+require "rbconfig"
+$LOAD_PATH.reject! { |path| path.start_with?(RbConfig::CONFIG["sitedir"]) }
 
+require "pathname"
 HOMEBREW_LIBRARY_PATH = Pathname(__dir__).parent.realpath.freeze
 
 require_relative "../utils/gems"

Library/Homebrew/utils/gems.rb

@@ -44,6 +44,9 @@ module Homebrew
   end
 
   def setup_gem_environment!(setup_path: true)
+    require "rubygems"
+    raise "RubyGems too old!" if Gem::Version.new(Gem::VERSION) < Gem::Version.new("2.2.0")
+
     # Match where our bundler gems are.
     gem_home = "#{HOMEBREW_LIBRARY_PATH}/vendor/bundle/ruby/#{RbConfig::CONFIG["ruby_version"]}"
     Gem.paths = {