2 files changed, 21 insertions, 3 deletions
diff --git a/activesupport/lib/active_support/callbacks.rb b/activesupport/lib/active_support/callbacks.rb
index 3a3061e536..7dd97eb1ab 100644
--- a/activesupport/lib/active_support/callbacks.rb
+++ b/activesupport/lib/active_support/callbacks.rb
@@ -4,6 +4,7 @@ require 'active_support/core_ext/array/extract_options'
 require 'active_support/core_ext/class/attribute'
 require 'active_support/core_ext/kernel/reporting'
 require 'active_support/core_ext/kernel/singleton_class'
+require 'active_support/core_ext/string/filters'
 require 'thread'
 
 module ActiveSupport
@@ -595,12 +596,23 @@ module ActiveSupport
         Proc.new do |target, result_lambda|
           terminate = true
           catch(:abort) do
-            result_lambda.call if result_lambda.is_a?(Proc)
-            terminate = false
+            result = result_lambda.call if result_lambda.is_a?(Proc)
+            if result == false
+              display_deprecation_warning_for_false_terminator
+            else
+              terminate = false
+            end
           end
           terminate
         end
       end
+
+      def display_deprecation_warning_for_false_terminator
+        ActiveSupport::Deprecation.warn(<<-MSG.squish)
+          Returning `false` in a callback will not implicitly halt a callback chain in the next release of Rails.
+          To explicitly halt a callback chain, please use `throw :abort` instead.
+        MSG
+      end
     end
 
     module ClassMethods
diff --git a/activesupport/lib/active_support/i18n_railtie.rb b/activesupport/lib/active_support/i18n_railtie.rb
index affcfb7398..9e742b1917 100644
--- a/activesupport/lib/active_support/i18n_railtie.rb
+++ b/activesupport/lib/active_support/i18n_railtie.rb
@@ -55,7 +55,13 @@ module I18n
 
       reloader = ActiveSupport::FileUpdateChecker.new(I18n.load_path.dup){ I18n.reload! }
       app.reloaders << reloader
-      ActionDispatch::Reloader.to_prepare { reloader.execute_if_updated }
+      ActionDispatch::Reloader.to_prepare do
+        reloader.execute_if_updated
+        # TODO: remove the following line as soon as the return value of
+        # callbacks is ignored, that is, returning `false` does not
+        # display a deprecation warning or halts the callback chain.
+        true
+      end
       reloader.execute
 
       @i18n_inited = true