Deprecate ActiveSupport::Inflector#acronym_regex

To be removed in Rails 6.0 (default for the deprecate helper).  Code
moved around as well for the ActiveSupport::Deprecation modules, since
it was dependent on ActiveSupport::Inflector being loaded for it to
work.  By "lazy loading" the Inflector code from within the Deprecation
code, we can require ActiveSupport::Deprecation from
ActiveSupport::Inflector and not get a circular dependency issue.

actionpack/test/dispatch/request_test.rb

@@ -763,7 +763,7 @@ class RequestMethod < BaseRequestTest
 
   test "post uneffected by local inflections" do
     existing_acronyms = ActiveSupport::Inflector.inflections.acronyms.dup
-    existing_acronym_regex = ActiveSupport::Inflector.inflections.acronym_regex.dup
+    assert_deprecated { ActiveSupport::Inflector.inflections.acronym_regex.dup }
     begin
       ActiveSupport::Inflector.inflections do |inflect|
         inflect.acronym "POS"

activesupport/lib/active_support/deprecation/constant_accessor.rb

 # frozen_string_literal: true
 
-require "active_support/inflector/methods"
-
 module ActiveSupport
   class Deprecation
     # DeprecatedConstantAccessor transforms a constant into a deprecated one by
@@ -29,6 +27,8 @@ class Deprecation
     #        ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
     module DeprecatedConstantAccessor
       def self.included(base)
+        require "active_support/inflector/methods"
+
         extension = Module.new do
           def const_missing(missing_const_name)
             if class_variable_defined?(:@@_deprecated_constants)

activesupport/lib/active_support/deprecation/proxy_wrappers.rb

 # frozen_string_literal: true
 
-require "active_support/inflector/methods"
 require "active_support/core_ext/regexp"
 
 module ActiveSupport
@@ -125,6 +124,8 @@ def warn(callstack, called, args)
     #        ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
     class DeprecatedConstantProxy < DeprecationProxy
       def initialize(old_const, new_const, deprecator = ActiveSupport::Deprecation.instance, message: "#{old_const} is deprecated! Use #{new_const} instead.")
+        require "active_support/inflector/methods"
+
         @old_const = old_const
         @new_const = new_const
         @deprecator = deprecator

activesupport/lib/active_support/inflector/inflections.rb

@@ -4,6 +4,7 @@
 require "active_support/core_ext/array/prepend_and_append"
 require "active_support/core_ext/regexp"
 require "active_support/i18n"
+require "active_support/deprecation"
 
 module ActiveSupport
   module Inflector
@@ -66,8 +67,9 @@ def self.instance(locale = :en)
         @__instance__[locale] ||= new
       end
 
-      attr_reader :plurals, :singulars, :uncountables, :humans,
-                  :acronyms, :acronym_regex
+      attr_reader :plurals, :singulars, :uncountables, :humans, :acronyms, :acronym_regex
+      deprecate :acronym_regex
+
       attr_reader :acronyms_camelize_regex, :acronyms_underscore_regex # :nodoc:
 
       def initialize

activesupport/test/inflector_test.rb

@@ -224,6 +224,12 @@ def test_underscore_acronym_sequence
     assert_equal("json_html_api", ActiveSupport::Inflector.underscore("JSONHTMLAPI"))
   end
 
+  def test_acronym_regexp_is_deprecated
+    assert_deprecated do
+      ActiveSupport::Inflector.inflections.acronym_regex
+    end
+  end
+
   def test_underscore
     CamelToUnderscore.each do |camel, underscore|
       assert_equal(underscore, ActiveSupport::Inflector.underscore(camel))