Avoid adding constants to Enumerable

Ref: aws/aws-sdk-ruby#2670

Some gems like aws-sdk-core use `Object#extend(Enumerable)`.
It's not a very good pattern, but it's somehwat handled ok by Ruby.

However if Enumerable has constants, then any time the module is
extended, the global constant cache is flushed and this has a very
negative impact on performance for the virtual machine, and even
worse for JITs.

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -1,13 +1,30 @@
 # frozen_string_literal: true
 
-module Enumerable
-  INDEX_WITH_DEFAULT = Object.new
-  private_constant :INDEX_WITH_DEFAULT
+module ActiveSupport
+  module EnumerableCoreExt # :nodoc:
+    module Constants
+      private
+        def const_missing(name)
+          if name == :SoleItemExpectedError
+            ::ActiveSupport::EnumerableCoreExt::SoleItemExpectedError
+          else
+            super
+          end
+        end
+    end
+  end
+end
 
+module Enumerable
   # Error generated by +sole+ when called on an enumerable that doesn't have
   # exactly one item.
   class SoleItemExpectedError < StandardError; end
 
+  # HACK: For performance reasons, Enumerable shouldn't have any constants of its own.
+  # So we move SoleItemExpectedError into ActiveSupport::EnumerableCoreExt.
+  ActiveSupport::EnumerableCoreExt::SoleItemExpectedError = remove_const(:SoleItemExpectedError)
+  singleton_class.prepend(ActiveSupport::EnumerableCoreExt::Constants)
+
   # Enumerable#sum was added in Ruby 2.4, but it only works with Numeric elements
   # when we omit an identity.
 
@@ -106,17 +123,17 @@ def index_by
   #
   #   %i( created_at updated_at ).index_with(Time.now)
   #   # => { created_at: 2020-03-09 22:31:47, updated_at: 2020-03-09 22:31:47 }
-  def index_with(default = INDEX_WITH_DEFAULT)
+  def index_with(default = (no_default = true))
     if block_given?
       result = {}
       each { |elem| result[elem] = yield(elem) }
       result
-    elsif default != INDEX_WITH_DEFAULT
+    elsif no_default
+      to_enum(:index_with) { size if respond_to?(:size) }
+    else
       result = {}
       each { |elem| result[elem] = default }
       result
-    else
-      to_enum(:index_with) { size if respond_to?(:size) }
     end
   end
 
@@ -240,8 +257,8 @@ def in_order_of(key, series)
   def sole
     case count
     when 1   then return first # rubocop:disable Style/RedundantReturn
-    when 0   then raise SoleItemExpectedError, "no item found"
-    when 2.. then raise SoleItemExpectedError, "multiple items found"
+    when 0   then raise ActiveSupport::EnumerableCoreExt::SoleItemExpectedError, "no item found"
+    when 2.. then raise ActiveSupport::EnumerableCoreExt::SoleItemExpectedError, "multiple items found"
     end
   end
 end

activesupport/test/core_ext/enumerable_test.rb

@@ -360,4 +360,13 @@ def test_sole
     assert_raise(expected_raise) { GenericEnumerable.new([1, 2]).sole }
     assert_raise(expected_raise) { GenericEnumerable.new([1, nil]).sole }
   end
+
+  def test_doesnt_bust_constant_cache
+    skip "Only applies to MRI" unless defined?(RubyVM.stat) && RubyVM.stat(:global_constant_state)
+
+    object = Object.new
+    assert_no_difference -> { RubyVM.stat(:global_constant_state) } do
+      object.extend(Enumerable)
+    end
+  end
 end