Deprecate `false` as the way to halt AS callbacks

After this commit, returning `false` in a callback will display a deprecation
warning to make developers aware of the fact that they need to explicitly
`throw(:abort)` if their intention is to halt a callback chain.

This commit also patches two internal uses of AS::Callbacks (inside
ActiveRecord and ActionDispatch) which sometimes return `false` but whose
returned value is not meaningful for the purpose of execution.

In both cases, the returned value is set to `true`, which does not affect the
execution of the callbacks but prevents unrequested deprecation warnings from
showing up.

1 files changed, 7 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index c39b045a5e..5045e3065e 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -206,7 +206,13 @@ module ActiveRecord
 
           if reflection.validate? && !method_defined?(validation_method)
             method = (collection ? :validate_collection_association : :validate_single_association)
-            define_non_cyclic_method(validation_method) { send(method, reflection) }
+            define_non_cyclic_method(validation_method) do
+              send(method, reflection)
+              # 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
             validate validation_method
           end
         end