irb: defer requiring rdoc to improve the start up time

lib/irb/cmd/help.rb

@@ -9,17 +9,18 @@
 #
 #
 
-require 'rdoc/ri/driver'
-
 require_relative "nop"
 
 # :stopdoc:
 module IRB
   module ExtendCommand
     class Help < Nop
-      begin
-        Ri = RDoc::RI::Driver.new
-      rescue SystemExit
+      def execute(*names)
+        require 'rdoc/ri/driver'
+        IRB::ExtendCommand::Help.const_set(:Ri, RDoc::RI::Driver.new)
+      rescue LoadError, SystemExit
+        IRB::ExtendCommand::Help.remove_method(:execute)
+        # raise NoMethodError in ensure
       else
         def execute(*names)
           if names.empty?
@@ -35,6 +36,8 @@ module IRB
           end
           nil
         end
+      ensure
+        execute(*names)
       end
     end
   end

lib/irb/completion.rb

@@ -8,7 +8,7 @@
 #
 
 require "readline"
-require "rdoc"
+autoload :RDoc, "rdoc"
 
 module IRB
   module InputCompletor # :nodoc:
@@ -267,8 +267,8 @@ module IRB
       end
     end
 
-    RDocRIDriver = RDoc::RI::Driver.new
     PerfectMatchedProc = ->(matched) {
+      RDocRIDriver ||= RDoc::RI::Driver.new
       if matched =~ /\A(?:::)?RubyVM/ and not ENV['RUBY_YES_I_AM_NOT_A_NORMAL_USER']
         File.open(File.join(__dir__, 'ruby_logo.aa')) do |f|
           RDocRIDriver.page do |io|