Deprecate the top-level HashWithIndifferentAccess contant

This constant was kept for the sake of backward compatibility; it
is still available under `ActiveSupport::HashWithIndifferentAccess`.

Furthermore, since Ruby 2.5 (https://bugs.ruby-lang.org/issues/11547)
won't support top level constant lookup, people would have to update
their code anyway.

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -50,7 +50,7 @@ def reload(*)
         super.tap do
           @previous_mutation_tracker = nil
           clear_mutation_trackers
-          @changed_attributes = HashWithIndifferentAccess.new
+          @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
         end
       end
 
@@ -70,13 +70,13 @@ def changes_internally_applied # :nodoc:
 
       def changes_applied
         @previous_mutation_tracker = mutation_tracker
-        @changed_attributes = HashWithIndifferentAccess.new
+        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
         clear_mutation_trackers
       end
 
       def clear_changes_information
         @previous_mutation_tracker = nil
-        @changed_attributes = HashWithIndifferentAccess.new
+        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
         forget_attribute_assignments
         clear_mutation_trackers
       end

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Deprecated the top level `HashWithIndifferentAccess` constant.
+
+    Only `ActiveSupport::HashWithIndifferentAccess` should be used now.
+
+    *Robin Dupret* (#27925)
+
 *   Deprecate `.halt_callback_chains_on_return_false`.
 
     *Rafael Mendonça França*

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -316,4 +316,20 @@ def set_defaults(target) # :doc:
   end
 end
 
-HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess
+class HashWithIndifferentAccess < ActiveSupport::HashWithIndifferentAccess
+  def initialize(*)
+    ActiveSupport::Deprecation.warn "HashWithIndifferentAccess is deprecated!" \
+                                    "Use ActiveSupport::HashWithIndifferentAccess instead."
+    super
+  end
+
+  def self.inherited(*)
+    ActiveSupport::Deprecation.warn "HashWithIndifferentAccess is deprecated!" \
+                                    "Use ActiveSupport::HashWithIndifferentAccess instead."
+    super
+  end
+
+  def encode_with(coder)
+    coder.represent_object(nil, ActiveSupport::HashWithIndifferentAccess.new(self))
+  end
+end

activesupport/test/core_ext/hash_ext_test.rb

@@ -8,6 +8,8 @@
 require "active_support/inflections"
 
 class HashExtTest < ActiveSupport::TestCase
+  HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess
+
   class IndifferentHash < ActiveSupport::HashWithIndifferentAccess
   end
 
@@ -1078,6 +1080,25 @@ def test_new_with_to_hash_conversion_copies_default_proc
     assert_equal 1, hash[:a]
     assert_equal 3, hash[:b]
   end
+
+  def test_top_level_hash_with_indifferent_access_is_deprecated
+    assert_deprecated do
+      ::HashWithIndifferentAccess.new
+    end
+  end
+
+  def test_top_level_hash_with_indifferent_access_can_be_extended
+    assert_deprecated do
+      Class.new(::HashWithIndifferentAccess)
+    end
+  end
+
+  def test_yaml_encoding_outputs_an_activesupport_namespaced_constant
+    ActiveSupport::Deprecation.silence do
+      instance = ::HashWithIndifferentAccess.new
+      assert_includes instance.to_yaml, "ActiveSupport::HashWithIndifferentAccess"
+    end
+  end
 end
 
 class IWriteMyOwnXML
@@ -1123,6 +1144,8 @@ def test_to_param_orders_by_key_in_ascending_order
 end
 
 class HashToXmlTest < ActiveSupport::TestCase
+  HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess
+
   def setup
     @xml_options = { root: :person, skip_instruct: true, indent: 0 }
   end