Apply instance delegators to application deprecators when available

This restores the 7.0 behavior of the instance delegator impacting all
deprecations from Rails.

activesupport/lib/active_support/deprecation/instance_delegator.rb

@@ -19,6 +19,7 @@ def method_added(method_name)
           use_instead =
             case method_name
             when :silence, :behavior=, :disallowed_behavior=, :disallowed_warnings=, :silenced=, :debug=
+              target = "(defined?(Rails.application.deprecators) ? Rails.application.deprecators : ActiveSupport::Deprecation.instance)"
               "Rails.application.deprecators.#{method_name}"
             when :warn, :deprecate_methods, :gem_name, :gem_name=, :deprecation_horizon, :deprecation_horizon=
               "your own Deprecation object"
@@ -26,9 +27,10 @@ def method_added(method_name)
               "Rails.application.deprecators[framework].#{method_name} where framework is for example :active_record"
             end
           args = /[^\]]=\z/.match?(method_name) ? "arg" : "..."
+          target ||= "ActiveSupport::Deprecation.instance"
           singleton_class.module_eval <<~RUBY, __FILE__, __LINE__ + 1
             def #{method_name}(#{args})
-              ActiveSupport::Deprecation.instance.#{method_name}(#{args})
+              #{target}.#{method_name}(#{args})
             ensure
               ActiveSupport.deprecator.warn("Calling #{method_name} on ActiveSupport::Deprecation is deprecated and will be removed from Rails (use #{use_instead} instead)")
             end

activesupport/test/deprecation_test.rb

@@ -779,6 +779,18 @@ def method
     end
   end
 
+  test "silence delegator is deprecated and delegates to the application's deprecators when available" do
+    with_rails_application_deprecators do
+      assert_deprecated("use Rails.application.deprecators.silence instead", ActiveSupport.deprecator) do
+        ActiveSupport::Deprecation.silence do
+          assert_not_deprecated(@deprecator) do
+            @deprecator.warn
+          end
+        end
+      end
+    end
+  end
+
   test "allow delegator is deprecated" do
     assert_deprecated("use Rails.application.deprecators[framework].allow", ActiveSupport.deprecator) do
       ActiveSupport::Deprecation.allow { }
@@ -797,6 +809,17 @@ def method
     ActiveSupport::Deprecation.instance.behavior = old_behavior
   end
 
+  test "behavior= delegator is deprecated and delegates to the application's deprecators when available" do
+    with_rails_application_deprecators do
+      called = false
+      assert_deprecated("use Rails.application.deprecators.behavior= instead", ActiveSupport.deprecator) do
+        ActiveSupport::Deprecation.behavior = ->(*) { called = true }
+        @deprecator.warn
+      end
+      assert called
+    end
+  end
+
   test "disallowed_behavior delegators are deprecated" do
     old_behavior = ActiveSupport::Deprecation.instance.disallowed_behavior
     assert_deprecated("use Rails.application.deprecators[framework].disallowed_behavior", ActiveSupport.deprecator) do
@@ -809,6 +832,18 @@ def method
     ActiveSupport::Deprecation.instance.disallowed_behavior = old_behavior
   end
 
+  test "disallowed_behavior= delegators is deprecated and delegates to the application's deprecators when available" do
+    with_rails_application_deprecators do
+      called = false
+      assert_deprecated("use Rails.application.deprecators.disallowed_behavior= instead", ActiveSupport.deprecator) do
+        ActiveSupport::Deprecation.disallowed_behavior = ->(*) { called = true }
+      end
+      @deprecator.disallowed_warnings = :all
+      @deprecator.warn
+      assert called
+    end
+  end
+
   test "debug delegators are deprecated" do
     old_debug = ActiveSupport::Deprecation.instance.debug
     assert_deprecated("use Rails.application.deprecators[framework].debug", ActiveSupport.deprecator) do
@@ -821,6 +856,15 @@ def method
     ActiveSupport::Deprecation.instance.debug = old_debug
   end
 
+  test "debug= delegator is deprecated and delegates to the application's deprecators when available" do
+    with_rails_application_deprecators do
+      assert_deprecated("use Rails.application.deprecators.debug= instead", ActiveSupport.deprecator) do
+        ActiveSupport::Deprecation.debug = true
+      end
+      assert @deprecator.debug
+    end
+  end
+
   test "silenced delegators are deprecated" do
     old_silenced = ActiveSupport::Deprecation.instance.silenced
     assert_deprecated("use Rails.application.deprecators[framework].silenced", ActiveSupport.deprecator) do
@@ -833,6 +877,15 @@ def method
     ActiveSupport::Deprecation.instance.silenced = old_silenced
   end
 
+  test "silenced= delegator is deprecated and delegates to the application's deprecators when available" do
+    with_rails_application_deprecators do
+      assert_deprecated("use Rails.application.deprecators.silenced= instead", ActiveSupport.deprecator) do
+        ActiveSupport::Deprecation.silenced = true
+      end
+      assert @deprecator.silenced
+    end
+  end
+
   test "disallowed_warnings delegators are deprecated" do
     old_disallowed_warnings = ActiveSupport::Deprecation.instance.disallowed_warnings
     assert_deprecated("use Rails.application.deprecators[framework].disallowed_warnings", ActiveSupport.deprecator) do
@@ -845,6 +898,15 @@ def method
     ActiveSupport::Deprecation.instance.disallowed_warnings = old_disallowed_warnings
   end
 
+  test "disallowed_warnings= delegator is deprecated and delegates to the application's deprecators when available" do
+    with_rails_application_deprecators do
+      assert_deprecated("use Rails.application.deprecators.disallowed_warnings= instead", ActiveSupport.deprecator) do
+        ActiveSupport::Deprecation.disallowed_warnings = :all
+      end
+      assert_equal :all, @deprecator.disallowed_warnings
+    end
+  end
+
   test "gem_name delegators are deprecated" do
     old_gem_name = ActiveSupport::Deprecation.instance.gem_name
     assert_deprecated("use your own Deprecation object instead", ActiveSupport.deprecator) do
@@ -881,6 +943,13 @@ def method_that_emits_deprecation(deprecator)
       deprecator.warn
     end
 
+    def with_rails_application_deprecators(&block)
+      application = Struct.new(:deprecators).new(ActiveSupport::Deprecation::Deprecators.new)
+      rails = Struct.new(:application).new(application)
+      rails.application.deprecators[:deprecator] = @deprecator
+      stub_const(Object, :Rails, rails, &block)
+    end
+
     def deprecator_with_messages
       klass = Class.new(ActiveSupport::Deprecation)
       deprecator = klass.new