Deprecator improvements

In preparation for removing deprecations for certain generators,
this commit documents the Deprecator and how to test deprecated
generators. It also adds a unit test using newly added matchers
based on Rails deprecation matchers.

With this change, we can remove generators in a follow up PR
while having the Deprecator covered and documented.

Author: stefannibrasil

lib/helpers/deprecator.rb

@@ -1,17 +1,70 @@
 # frozen_string_literal: true
 
 # Based on Rails ActiveSupport Deprecator
-# https://github.com/rails/rails/blob/6f0d1ad14b92b9f5906e44740fce8b4f1c7075dc/activesupport/lib/active_support/deprecation/constant_accessor.rb
+# https://github.com/rails/rails/blob/main/activesupport/lib/active_support/deprecation/constant_accessor.rb
 
 # rubocop:disable Style/ClassVars
 module Faker
+  # Provides a way to rename generators, including their namespaces, with a deprecation cycle in which
+  # both the old and new names work, but using the old one prints a deprecation message.
+  #
+  # Deprecator provides a deprecate_generator method to be used when
+  # renaming a generator. For example, let's say we want to change the following Generator's
+  # name to <tt>Faker::NewGenerator</tt>:
+  #
+  #   module Faker
+  #     class Generator
+  #       def self.generate
+  #         "be kind"
+  #       end
+  #     end
+  #   end
+  #
+  # To rename it, you need to do the update the name and declare the deprecation by
+  # including the <tt>Deprecator</tt> module and using the deprecate_generator method:
+  #
+  #   module Faker
+  #     class NewGenerator
+  #       def self.generate
+  #         "be kind"
+  #       end
+  #     end
+  #
+  #     include Deprecator
+  #     deprecate_generator('DeprecatedGenerator', NewGenerator)
+  #   end
+  #
+  # The first argument is a constant name (no colons) as a string. It is the name of
+  # the constant you want to deprecate.
+  #
+  # The second argument is the constant path of the replacement (no colons) as a constant.
+  #
+  # For this to work, a +const_missing+ hook is installed. When users
+  # reference the deprecated constant, the callback prints the
+  # message and constantizes the replacement.
+  #
+  # With that in place, references to <tt>Faker::Deprecator</tt> still work, they
+  # evaluate to <tt>Faker::NewGenerator</tt> now, and trigger a deprecation warning:
+  #
+  #   Faker::Generator.generate
+  #   # DEPRECATION WARNING: Faker::Generator is deprecated. Use Faker::NewGenerator instead
+  #   # "be kind"
+  #
+  # For testing the deprecations, we provide <tt>assert_deprecated</tt>
+  # and <tt>assert_not_deprecated</tt> matchers.
+  #
+  # There's also a <tt>Faker::Deprecator.skip_warning</tt> helper to silence
+  # the deprecation messages in the *test* output. Use it for generators that have lots of tests
+  # to avoid too many noise when running the tests.
   module Deprecator
     def self.included(base)
       extension = Module.new do
         def const_missing(missing_const_name)
           if class_variable_defined?(:@@_deprecated_constants) && (replacement = class_variable_get(:@@_deprecated_constants)[missing_const_name.to_s])
             unless Faker::Deprecator.skip_warning?
-              $stdout.puts("DEPRECATION WARNING: #{name}::#{replacement[:old_generator]} is deprecated. Use #{replacement[:new_constant]} instead.")
+              deprecated_message = "#{name}::#{replacement[:old_generator]} is deprecated."
+              replacement_message = "Use #{replacement[:new_constant]} instead."
+              $stdout.puts("DEPRECATION WARNING: #{deprecated_message} #{replacement_message}")
             end
 
             return replacement[:new_constant]
@@ -22,13 +75,25 @@ def const_missing(missing_const_name)
 
         def deprecate_generator(old_generator_name, new_generator_constant)
           class_variable_set(:@@_deprecated_constants, {}) unless class_variable_defined?(:@@_deprecated_constants)
-          class_variable_get(:@@_deprecated_constants)[old_generator_name] = { new_constant: new_generator_constant, old_generator: old_generator_name }
+          class_variable_get(:@@_deprecated_constants)[old_generator_name] = {
+            new_constant: new_generator_constant,
+            old_generator: old_generator_name
+          }
         end
       end
 
       base.singleton_class.prepend extension
     end
 
+    # Silence deprecation warnings within the block.
+    #
+    #   Faker::Generator.generate
+    #   # => DEPRECATION WARNING: Faker::Generator is deprecated. Use Faker::NewGenerator instead.
+    #
+    #   Faker::Deprecator.skip_warning do
+    #     Faker::Generator.generate
+    #   end
+    #   # => nil
     def self.skip_warning
       original = Faker::Deprecator.skip
       Faker::Deprecator.skip = true

test/helpers/test_faker_deprecator.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+
+class TestFakerDeprecation < Test::Unit::TestCase
+  def test_using_a_deprecated_generator_returns_a_warning_message
+    assert_deprecated do
+      Faker::Dogs.say
+    end
+
+    assert_equal 'meow', Faker::Dogs.say
+  end
+
+  def test_using_a_non_deprecated_generator_does_not_return_a_warning_message
+    assert_not_deprecated do
+      Faker::Cats.say
+    end
+    assert_equal 'meow', Faker::Cats.say
+  end
+
+  def test_testing_a_deprecated_generator_with_skip_warning_does_not_return_a_warning_message
+    actual_stdout, actual_stderr = capture_output do
+      Faker::Deprecator.skip_warning do
+        Faker::Dogs.say
+      end
+    end
+
+    assert_empty(actual_stdout)
+    assert_empty(actual_stderr)
+    assert_equal 'meow', Faker::Dogs.say
+  end
+
+  def test_deprecated_with_skip_warning_does_not_generate_message
+    Faker::Deprecator.skip_warning do
+      assert_not_deprecated do
+        Faker::Dogs.say
+      end
+    end
+
+    assert_equal 'meow', Faker::Dogs.say
+  end
+end
+
+module Faker
+  class Cats < Base
+    def self.say
+      'meow'
+    end
+  end
+
+  include Faker::Deprecator
+  deprecate_generator('Dogs', Cats)
+end

test/support/deprecation.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+# Based on Rails Testing Deprecator
+# https://github.com/rails/rails/blob/main/activesupport/lib/active_support/testing/deprecation.rb
+
+# Asserts that a matching deprecation warning was emitted during the execution of the yielded block.
+#
+#   assert_deprecated do
+#     DeprecatedGenerator.generate
+#   end
+#
+def assert_deprecated(&block)
+  warning = with_captured_stdout(&block)
+  result = yield block
+
+  refute_predicate warning, :empty?, 'Expected a deprecation warning within the block but received none'
+
+  result
+end
+
+# Asserts that no deprecation warnings are emitted during the execution of the yielded block.
+#
+#   assert_not_deprecated do
+#     Faker::Internet.email
+#   end
+def assert_not_deprecated(&block)
+  warning = with_captured_stdout(&block)
+  result = yield block
+
+  assert_predicate warning, :empty?, "Expected no deprecation warning within the block but received a deprecation: #{warning}"
+  result
+end
+
+def with_captured_stdout(&block)
+  original_stdout = $stdout
+  $stdout = StringIO.new
+  yield block
+  $stdout.string
+ensure
+  $stdout = original_stdout
+end

test/test_faker_deprecator.rb

@@ -11,6 +11,7 @@
 
 require_relative 'support/assert_not_english'
 require_relative 'support/assert_email_regex'
+require_relative 'support/deprecation'
 require 'minitest/autorun'
 require 'test/unit'
 require 'rubygems'