Deprecate `false` as the way to halt AM validation callbacks

Before this commit, returning `false` in an ActiveModel validation
callback such as `before_validation` would halt the callback chain.

After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.

The preferred way to halt a callback chain is to explicitly `throw(:abort)`.

1 files changed, 2 insertions, 3 deletions

diff --git a/activemodel/lib/active_model/validations/callbacks.rb b/activemodel/lib/active_model/validations/callbacks.rb
index 449f1a0299..4b58ef66e3 100644
--- a/activemodel/lib/active_model/validations/callbacks.rb
+++ b/activemodel/lib/active_model/validations/callbacks.rb
@@ -15,15 +15,14 @@ module ActiveModel
     #     after_validation  :do_stuff_after_validation
     #   end
     #
-    # Like other <tt>before_*</tt> callbacks if +before_validation+ returns
-    # +false+ then <tt>valid?</tt> will not be called.
+    # Like other <tt>before_*</tt> callbacks if +before_validation+ throws
+    # +:abort+ then <tt>valid?</tt> will not be called.
     module Callbacks
       extend ActiveSupport::Concern
 
       included do
         include ActiveSupport::Callbacks
         define_callbacks :validation,
-                         terminator: ->(_,result_lambda) { result_lambda.call == false },
                          skip_after_callbacks_if_terminated: true,
                          scope: [:kind, :name]
       end