Speed up loading psych gem

Rakefile

@@ -29,6 +29,8 @@ if RUBY_PLATFORM =~ /java/
 else
   require 'rake/extensiontask'
   spec = Gem::Specification.load("psych.gemspec")
+  # Add stub/ on $LOAD_PATH only for binary gems
+  spec.require_paths.insert(0, *%w[stub])
   Rake::ExtensionTask.new("psych", spec) do |ext|
     ext.lib_dir = File.join(*['lib', ENV['FAT_DIR']].compact)
     ext.cross_compile = true

lib/psych.rb

@@ -1,21 +1,22 @@
 # frozen_string_literal: true
 require 'psych/versions'
-case RUBY_ENGINE
-when 'jruby'
-  require 'psych_jars'
-  if JRuby::Util.respond_to?(:load_ext)
-    JRuby::Util.load_ext('org.jruby.ext.psych.PsychLibrary')
+
+# When required from the stub file, the C extension is already loaded
+unless caller_locations(1, 1)[0].absolute_path == File.expand_path('../stub/psych.rb', __dir__)
+  case RUBY_ENGINE
+  when 'jruby'
+    require 'psych_jars'
+    if JRuby::Util.respond_to?(:load_ext)
+      JRuby::Util.load_ext('org.jruby.ext.psych.PsychLibrary')
+    else
+      require 'java'; require 'jruby'
+      org.jruby.ext.psych.PsychLibrary.new.load(JRuby.runtime, false)
+    end
   else
-    require 'java'; require 'jruby'
-    org.jruby.ext.psych.PsychLibrary.new.load(JRuby.runtime, false)
-  end
-else
-  begin
-    require "#{RUBY_VERSION[/\d+\.\d+/]}/psych.so"
-  rescue LoadError
     require 'psych.so'
   end
 end
+
 require 'psych/nodes'
 require 'psych/streaming'
 require 'psych/visitors'

psych.gemspec

@@ -41,7 +41,8 @@ DESCRIPTION
     "lib/psych/set.rb", "lib/psych/stream.rb", "lib/psych/streaming.rb", "lib/psych/syntax_error.rb",
     "lib/psych/tree_builder.rb", "lib/psych/versions.rb", "lib/psych/visitors.rb","lib/psych/visitors/depth_first.rb",
     "lib/psych/visitors/emitter.rb", "lib/psych/visitors/json_tree.rb", "lib/psych/visitors/to_ruby.rb",
-    "lib/psych/visitors/visitor.rb", "lib/psych/visitors/yaml_tree.rb", "lib/psych/y.rb", "psych.gemspec"
+    "lib/psych/visitors/visitor.rb", "lib/psych/visitors/yaml_tree.rb", "lib/psych/y.rb", "psych.gemspec",
+    "stub/psych.rb"
   ]
 
   s.rdoc_options = ["--main", "README.md"]

stub/psych.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+# This file is only on $LOAD_PATH when using a binary gem
+
+require "#{RUBY_VERSION[/\d+\.\d+/]}/psych.so"
+require_relative '../lib/psych'