Add Timecop cop

This cop makes `Timecop` illegal, in favour of
`ActiveSupport::Testing::TimeHelpers`.

Specifically,

- `Timecop.freeze` should be replaced with `freeze_time` (autocorrected)
- `Timecop.freeze(...)` should be replaced with `travel` or `travel_to`
- `Timecop.return` should be replaced with `travel_back` (autocorrected)
- `Timecop.travel` should be replaced with `travel` or `travel_to`.
  - Explicitly travelling again should be used instead of relying on time continuing to flow
- `Timecop` should not appear anywhere

lib/rubocop/cop/rails/timecop.rb

@@ -0,0 +1,102 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      class Timecop < Cop
+        FREEZE_MESSAGE = 'Use `freeze_time` instead of `Timecop.freeze`'
+        FREEZE_WITH_ARGUMENTS_MESSAGE = 'Use `travel` or `travel_to` instead of `Timecop.freeze`'
+        RETURN_MESSAGE = 'Use `travel_back` instead of `Timecop.return`'
+        TRAVEL_MESSAGE = 'Use `travel` or `travel_to` instead of `Timecop.travel`. If you need time to keep flowing, ' \
+          'simulate it by travelling again.'
+        MSG = 'Use `ActiveSupport::Testing::TimeHelpers` instead of `Timecop`'
+
+        FREEZE_TIME = 'freeze_time'
+        TRAVEL_BACK = 'travel_back'
+
+        TIMECOP_PATTERN_STRING = <<~PATTERN
+          (const {nil? (:cbase)} :Timecop)
+        PATTERN
+
+        def_node_matcher :timecop, TIMECOP_PATTERN_STRING
+
+        def_node_matcher :timecop_send, <<~PATTERN
+          (send
+            #{TIMECOP_PATTERN_STRING} ${:freeze :return :travel}
+            $...
+          )
+        PATTERN
+
+        def on_const(node)
+          return unless timecop(node)
+
+          timecop_send(node.parent) do |message, arguments|
+            return on_timecop_send(node.parent, message, arguments)
+          end
+
+          add_offense(node)
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            timecop_send(node) do |message, arguments|
+              case message
+              when :freeze
+                autocorrect_freeze(corrector, node, arguments)
+              when :return
+                autocorrect_return(corrector, node, arguments)
+              end
+            end
+          end
+        end
+
+        private
+
+        def on_timecop_send(node, message, arguments)
+          case message
+          when :freeze
+            on_timecop_freeze(node, arguments)
+          when :return
+            on_timecop_return(node, arguments)
+          when :travel
+            on_timecop_travel(node, arguments)
+          else
+            add_offense(node)
+          end
+        end
+
+        def on_timecop_freeze(node, arguments)
+          if arguments.empty?
+            add_offense(node, message: FREEZE_MESSAGE)
+          else
+            add_offense(node, message: FREEZE_WITH_ARGUMENTS_MESSAGE)
+          end
+        end
+
+        def on_timecop_return(node, _arguments)
+          add_offense(node, message: RETURN_MESSAGE)
+        end
+
+        def on_timecop_travel(node, _arguments)
+          add_offense(node, message: TRAVEL_MESSAGE)
+        end
+
+        def autocorrect_freeze(corrector, node, arguments)
+          return unless arguments.empty?
+
+          corrector.replace(receiver_and_message_range(node), FREEZE_TIME)
+        end
+
+        def autocorrect_return(corrector, node, _arguments)
+          corrector.replace(receiver_and_message_range(node), TRAVEL_BACK)
+        end
+
+        def receiver_and_message_range(node)
+          # FIXME: There is probably a better way to do this
+          # Just trying to get the range of `Timecop.method_name`, without args, or block, or anything
+          node.location.expression.with(end_pos: node.location.selector.end_pos)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -55,6 +55,7 @@ module Cop
 require_relative 'rails/scope_args'
 require_relative 'rails/skips_model_validations'
 require_relative 'rails/time_zone'
+require_relative 'rails/timecop'
 require_relative 'rails/uniq_before_pluck'
 require_relative 'rails/unknown_env'
 require_relative 'rails/validation'

spec/rubocop/cop/rails/timecop_spec.rb

@@ -0,0 +1,132 @@
+# frozen_string_literal: true
+
+RSpec.describe(RuboCop::Cop::Rails::Timecop, :config) do
+  subject(:cop) { described_class.new(config) }
+
+  describe 'Timecop.freeze' do
+    context 'without a block' do
+      context 'without arguments' do
+        it 'adds an offense' do
+          expect_offense(<<~RUBY)
+            Timecop.freeze
+            ^^^^^^^^^^^^^^ Use `freeze_time` instead of `Timecop.freeze`
+          RUBY
+        end
+
+        it 'autocorrects to `freeze_time`' do
+          expect(autocorrect_source('Timecop.freeze')).to(eq('freeze_time'))
+        end
+      end
+
+      context 'with arguments' do
+        it 'adds an offense' do
+          expect_offense(<<~RUBY)
+            Timecop.freeze(123)
+            ^^^^^^^^^^^^^^^^^^^ Use `travel` or `travel_to` instead of `Timecop.freeze`
+          RUBY
+        end
+
+        it 'does not autocorrect' do
+          source = 'Timecop.freeze(123)'
+
+          expect(autocorrect_source(source)).to(eq(source))
+        end
+      end
+    end
+
+    context 'with a block' do
+      context 'without arguments' do
+        it 'adds an offense' do
+          expect_offense(<<~RUBY)
+            Timecop.freeze { }
+            ^^^^^^^^^^^^^^ Use `freeze_time` instead of `Timecop.freeze`
+          RUBY
+        end
+
+        it 'autocorrects to `freeze_time`' do
+          expect(autocorrect_source('Timecop.freeze { }')).to(eq('freeze_time { }'))
+        end
+      end
+
+      context 'with arguments' do
+        it 'adds an offense' do
+          expect_offense(<<~RUBY)
+            Timecop.freeze(123) { }
+            ^^^^^^^^^^^^^^^^^^^ Use `travel` or `travel_to` instead of `Timecop.freeze`
+          RUBY
+        end
+
+        # FIXME: Is this how NOT autocorrecting something should be tested?
+        it 'does not autocorrect' do
+          source = 'Timecop.freeze(123) { }'
+
+          expect(autocorrect_source(source)).to(eq(source))
+        end
+      end
+    end
+  end
+
+  describe 'Timecop.return' do
+    context 'without a block' do
+      it 'adds an offense' do
+        expect_offense(<<~RUBY)
+          Timecop.return
+          ^^^^^^^^^^^^^^ Use `travel_back` instead of `Timecop.return`
+        RUBY
+      end
+
+      it 'autocorrects to `travel_back`' do
+        expect(autocorrect_source('Timecop.return')).to(eq('travel_back'))
+      end
+    end
+
+    context 'with a block' do
+      it 'adds an offense' do
+        expect_offense(<<~RUBY)
+          Timecop.return { }
+          ^^^^^^^^^^^^^^ Use `travel_back` instead of `Timecop.return`
+        RUBY
+      end
+
+      it 'autocorrects to `travel_back`' do
+        expect(autocorrect_source('Timecop.return { }')).to(eq('travel_back { }'))
+      end
+    end
+  end
+
+  describe 'Timecop.travel' do
+    it 'adds an offense' do
+      expect_offense(<<~RUBY)
+        Timecop.travel(123) { }
+        ^^^^^^^^^^^^^^^^^^^ Use `travel` or `travel_to` instead of `Timecop.travel`. If you need time to keep flowing, simulate it by travelling again.
+      RUBY
+    end
+  end
+
+  describe 'Timecop.*' do
+    it 'adds an offense' do
+      expect_offense(<<~RUBY)
+        Timecop.foo
+        ^^^^^^^ Use `ActiveSupport::Testing::TimeHelpers` instead of `Timecop`
+      RUBY
+    end
+  end
+
+  describe 'Timecop' do
+    it 'adds an offense' do
+      expect_offense(<<~RUBY)
+        Timecop.foo
+        ^^^^^^^ Use `ActiveSupport::Testing::TimeHelpers` instead of `Timecop`
+      RUBY
+    end
+  end
+
+  describe '::Timecop' do
+    it 'adds an offense' do
+      expect_offense(<<~RUBY)
+        ::Timecop.foo
+        ^^^^^^^^^ Use `ActiveSupport::Testing::TimeHelpers` instead of `Timecop`
+      RUBY
+    end
+  end
+end