Improvements to deprecate_constant

Author: fxn

activesupport/lib/active_support/deprecation/constant_accessor.rb

@@ -2,28 +2,6 @@
 
 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 a deprecator.
-    #
-    # 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', deprecator: ActiveSupport::Deprecation.new
-    #
-    #   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)
         require "active_support/inflector/methods"
@@ -39,9 +17,53 @@ def const_missing(missing_const_name)
             super
           end
 
-          def deprecate_constant(const_name, new_constant, deprecator:, message: nil)
+          # Provides a way to rename constants with a deprecation cycle in which
+          # both the old and new names work, but using the old one prints a
+          # deprecation message.
+          #
+          # In order to rename <tt>A::B</tt> to <tt>C::D</tt>, you need to delete the
+          # definition of <tt>A::B</tt> and declare the deprecation in +A+:
+          #
+          #   require "active_support/deprecation"
+          #
+          #   module A
+          #     include ActiveSupport::Deprecation::DeprecatedConstantAccessor
+          #
+          #     deprecate_constant "B", "C::D", deprecator: ActiveSupport::Deprecation.new
+          #   end
+          #
+          # The first argument is a constant name (no colons). It is the name of
+          # the constant you want to deprecate in the enclosing class or module.
+          #
+          # The second argument is the constant path of the replacement. That
+          # has to be a full path even if the replacement is defined in the same
+          # namespace as the deprecated one was.
+          #
+          # In both cases, strings and symbols are supported.
+          #
+          # The +deprecator+ keyword argument is the object that will print the
+          # deprecation message, an instance of ActiveSupport::Deprecation.
+          #
+          # With that in place, client code referencing <tt>A::B</tt> will see
+          #
+          #   DEPRECATION WARNING: A::B is deprecated! Use C::D instead.
+          #   (called from ...)
+          #
+          # The message can be customized with the optional +message+ keyword
+          # argument.
+          #
+          # For this to work, a +const_missing+ hook is installed. When client
+          # code references the deprecated constant, the callback prints the
+          # message and constantizes the replacement.
+          #
+          # Caveat: If the deprecated constant name is reachable in a different
+          # namespace and Ruby constant lookup finds it, the hook won't be
+          # called and the deprecation won't work as intended. This may happen,
+          # for example, if an ancestor of the enclosing namespace has a
+          # constant with the same name. This is an unsupported edge case.
+          def deprecate_constant(old_constant_name, new_constant_path, deprecator:, message: nil)
             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 }
+            class_variable_get(:@@_deprecated_constants)[old_constant_name.to_s] = { new: new_constant_path, message: message, deprecator: deprecator }
           end
         end
         base.singleton_class.prepend extension