Deprecate `false` as the way to halt AM callbacks

Before this commit, returning `false` in an ActiveModel `before_` callback
such as `before_create` 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)`.

activemodel/CHANGELOG.md

@@ -1,6 +1,6 @@
 *   Deprecate returning `false` as a way to halt callback chains.
 
-    Returning `false` in a `before_` validation callback will display a
+    Returning `false` in a `before_` callback will display a
     deprecation warning explaining that the preferred method to halt a callback
     chain is to explicitly `throw(:abort)`.
 

activemodel/lib/active_model/callbacks.rb

@@ -6,7 +6,7 @@ module ActiveModel
   # Provides an interface for any class to have Active Record like callbacks.
   #
   # Like the Active Record methods, the callback chain is aborted as soon as
-  # one of the methods in the chain returns +false+.
+  # one of the methods throws +:abort+.
   #
   # First, extend ActiveModel::Callbacks from the class you are creating:
   #
@@ -103,7 +103,6 @@ module ActiveModel
     def define_model_callbacks(*callbacks)
       options = callbacks.extract_options!
       options = {
-        terminator: ->(_,result_lambda) { result_lambda.call == false },
         skip_after_callbacks_if_terminated: true,
         scope: [:kind, :name],
         only: [:before, :around, :after]

activemodel/test/cases/callbacks_test.rb

@@ -33,11 +33,13 @@ class CallbacksTest < ActiveModel::TestCase
     def initialize(options = {})
       @callbacks = []
       @valid = options[:valid]
-      @before_create_returns = options[:before_create_returns]
+      @before_create_returns = options.fetch(:before_create_returns, true)
+      @before_create_throws = options[:before_create_throws]
     end
 
     def before_create
       @callbacks << :before_create
+      throw(@before_create_throws) if @before_create_throws
       @before_create_returns
     end
 
@@ -62,10 +64,18 @@ class CallbacksTest < ActiveModel::TestCase
     assert_equal model.callbacks.last, :final_callback
   end
 
-  test "the callback chain is halted when a before callback returns false" do
+  test "the callback chain is halted when a before callback returns false (deprecated)" do
     model = ModelCallbacks.new(before_create_returns: false)
+    assert_deprecated do
+      model.create
+      assert_equal model.callbacks.last, :before_create
+    end
+  end
+
+  test "the callback chain is halted when a callback throws :abort" do
+    model = ModelCallbacks.new(before_create_throws: :abort)
     model.create
-    assert_equal model.callbacks.last, :before_create
+    assert_equal model.callbacks, [:before_create]
   end
 
   test "after callbacks are not executed if the block returns false" do