Added force_activate feature again

Merged: https://github.com/ruby/ruby/pull/12624

lib/bundled_gems.rb

@@ -218,6 +218,38 @@ module Gem::BUNDLED_GEMS # :nodoc:
 
     msg
   end
+
+  def self.force_activate(gem)
+    Bundler.reset!
+
+    builder = Bundler::Dsl.new
+    if Bundler.definition.gemfiles.empty? # bundler/inline
+      Bundler.definition.locked_gems.specs.each{|spec| builder.gem spec.name, spec.version.to_s }
+    else
+      Bundler.definition.gemfiles.each{|gemfile| builder.eval_gemfile(gemfile) }
+    end
+    builder.gem gem
+
+    definition = builder.to_definition(nil, true)
+    definition.validate_runtime!
+
+    begin
+      orig_ui = Bundler.ui
+      orig_no_lock = Bundler::Definition.no_lock
+
+      ui = Bundler::UI::Shell.new
+      ui.level = "silent"
+      Bundler.ui = ui
+      Bundler::Definition.no_lock = true
+
+      Bundler::Runtime.new(nil, definition).setup
+    rescue Bundler::GemNotFound
+      warn "Failed to activate #{gem}, please install it with 'gem install #{gem}'"
+    ensure
+      Bundler.ui = orig_ui
+      Bundler::Definition.no_lock = orig_no_lock
+    end
+  end
 end
 
 # for RubyGems without Bundler environment.

prelude.rb

@@ -1,7 +1,12 @@
 class Binding
   # :nodoc:
   def irb
-    require 'irb'
+    begin
+      require 'irb'
+    rescue LoadError, Gem::LoadError
+      Gem::BUNDLED_GEMS.force_activate 'irb'
+      retry
+    end
     irb
   end