Add AS::Deprecation::DeprecatedConstantAccessor

An alternative to DeprecatedConstantProxy which works more transparently
with exceptions because it returns the object that the new constant
refers to rather than a proxy. This is then compatible with
`rescue OldException`.

activesupport/CHANGELOG.md

+* Add ActiveSupport::Deprecation::DeprecatedConstantAccessor
+
+  Provides transparent deprecation of constants, compatible with exceptions.
+  Example usage:
+
+      module Example
+        include ActiveSupport::Deprecation::DeprecatedConstantAccessor
+        deprecate_constant 'OldException', 'Elsewhere::NewException'
+      end
+
+  *Dominic Cleal*
+
 Please check [5-1-stable](https://github.com/rails/rails/blob/5-1-stable/activesupport/CHANGELOG.md) for previous changes.

activesupport/lib/active_support/deprecation.rb

@@ -15,6 +15,7 @@ class Deprecation
     require "active_support/deprecation/instance_delegator"
     require "active_support/deprecation/behaviors"
     require "active_support/deprecation/reporting"
+    require "active_support/deprecation/constant_accessor"
     require "active_support/deprecation/method_wrappers"
     require "active_support/deprecation/proxy_wrappers"
     require "active_support/core_ext/module/deprecation"

activesupport/lib/active_support/deprecation/constant_accessor.rb

+require "active_support/inflector/methods"
+
+module ActiveSupport
+  class Deprecation
+    # DeprecatedConstantAccessor transforms a constant into a deprecated one by
+    # hooking +const_missing+.
+    #
+    # It takes the names of an old (deprecated) constant and of a new constant
+    # (both in string form) and optionally a deprecator. The deprecator defaults
+    # to +ActiveSupport::Deprecator+ if none is specified.
+    #
+    # The deprecated constant now returns the same object as the new one rather
+    # than a proxy object, so it can be used transparently in +rescue+ blocks
+    # etc.
+    #
+    #   PLANETS = %w(mercury venus earth mars jupiter saturn uranus neptune pluto)
+    #
+    #   (In a later update, the original implementation of `PLANETS` has been removed.)
+    #
+    #   PLANETS_POST_2006 = %w(mercury venus earth mars jupiter saturn uranus neptune)
+    #   include ActiveSupport::Deprecation::DeprecatedConstantAccessor
+    #   deprecate_constant 'PLANETS', 'PLANETS_POST_2006'
+    #
+    #   PLANETS.map { |planet| planet.capitalize }
+    #   # => DEPRECATION WARNING: PLANETS is deprecated! Use PLANETS_POST_2006 instead.
+    #        (Backtrace information…)
+    #        ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
+    module DeprecatedConstantAccessor
+      def self.included(base)
+        extension = Module.new do
+          def const_missing(missing_const_name)
+            if class_variable_defined?(:@@_deprecated_constants)
+              if (replacement = class_variable_get(:@@_deprecated_constants)[missing_const_name.to_s])
+                replacement[:deprecator].warn(replacement[:message] || "#{name}::#{missing_const_name} is deprecated! Use #{replacement[:new]} instead.", caller_locations)
+                return ActiveSupport::Inflector.constantize(replacement[:new].to_s)
+              end
+            end
+            super
+          end
+
+          def deprecate_constant(const_name, new_constant, message: nil, deprecator: ActiveSupport::Deprecation.instance)
+            class_variable_set(:@@_deprecated_constants, {}) unless class_variable_defined?(:@@_deprecated_constants)
+            class_variable_get(:@@_deprecated_constants)[const_name.to_s] = { new: new_constant, message: message, deprecator: deprecator }
+          end
+        end
+        base.singleton_class.prepend extension
+      end
+    end
+  end
+end

activesupport/test/deprecation_test.rb

@@ -35,6 +35,18 @@ module B
   A = ActiveSupport::Deprecation::DeprecatedConstantProxy.new("Deprecatee::A", "Deprecatee::B::C")
 end
 
+class DeprecateeWithAccessor
+  include ActiveSupport::Deprecation::DeprecatedConstantAccessor
+
+  module B
+    C = 1
+  end
+  deprecate_constant "A", "DeprecateeWithAccessor::B::C"
+
+  class NewException < StandardError; end
+  deprecate_constant "OldException", "DeprecateeWithAccessor::NewException"
+end
+
 class DeprecationTest < ActiveSupport::TestCase
   include ActiveSupport::Testing::Stream
 
@@ -162,6 +174,17 @@ def test_deprecated_constant_proxy
     assert_not_deprecated { assert_equal Deprecatee::B::C.class, Deprecatee::A.class }
   end
 
+  def test_deprecated_constant_accessor
+    assert_not_deprecated { DeprecateeWithAccessor::B::C }
+    assert_deprecated("DeprecateeWithAccessor::A") { assert_equal DeprecateeWithAccessor::B::C, DeprecateeWithAccessor::A }
+  end
+
+  def test_deprecated_constant_accessor_exception
+    raise DeprecateeWithAccessor::NewException.new("Test")
+  rescue DeprecateeWithAccessor::OldException => e
+    assert_kind_of DeprecateeWithAccessor::NewException, e
+  end
+
   def test_assert_deprecated_raises_when_method_not_deprecated
     assert_raises(Minitest::Assertion) { assert_deprecated { @dtc.not } }
   end