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` or `unfreeze_time` (autocorrected)
- `Timecop.scale` & `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

CHANGELOG.md

@@ -852,3 +852,4 @@
 [@john-h-k]: https://github.com/john-h-k
 [@DRBragg]: https://github.com/DRBragg
 [@lucthev]: https://github.com/lucthev
+[@sambostock]: https://github.com/sambostock

changelog/new_add_timecop_cop.md

@@ -0,0 +1 @@
+* [#38](https://github.com/rubocop/rubocop-rails/pull/38): Add `Timecop` cop. ([@sambostock][])

config/default.yml

@@ -1032,6 +1032,12 @@ Rails/TimeZoneAssignment:
     - spec/**/*.rb
     - test/**/*.rb
 
+Rails/Timecop:
+  Description: 'Prefer `ActiveSupport::Testing::TimeHelpers` over `Timecop`.'
+  Enabled: pending
+  VersionAdded: <<next>>
+  SafeAutoCorrect: false
+
 Rails/ToFormattedS:
   Description: 'Checks for consistent uses of `to_fs` or `to_formatted_s`.'
   StyleGuide: 'https://rails.rubystyle.guide/#prefer-to-fs'

lib/rubocop/cop/rails/timecop.rb

@@ -0,0 +1,198 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Disallows all usage of `Timecop`, in favour of
+      # `ActiveSupport::Testing::TimeHelpers`.
+      #
+      # ## Migration
+      # `Timecop.freeze` should be replaced with `freeze_time` when used
+      # without arguments. Where a `duration` has been passed to `freeze`, it
+      # should be replaced with `travel`. Likewise, where a `time` has been
+      # passed to `freeze`, it should be replaced with `travel_to`.
+      #
+      # `Timecop.scale` should be replaced by explicitly calling `travel` or
+      # `travel_to` with the expected `durations` or `times`, respectively,
+      # rather than relying on allowing time to continue to flow.
+      #
+      # `Timecop.return` should be replaced with `travel_back`, when used
+      # without a block. `travel_back` does not accept a block, so where
+      # `return` is used with a block, it should be replaced by explicitly
+      # calling `freeze_time` with a block, and passing the `time` to
+      # temporarily return to.
+      #
+      # `Timecop.travel` should be replaced by `travel` or `travel_to` when
+      # passed a `duration` or `time`, respectively. As with `Timecop.scale`,
+      # rather than relying on time continuing to flow, it should be travelled
+      # to explicitly.
+      #
+      # All other usages of `Timecop` are similarly disallowed.
+      #
+      # ## RSpec Caveats
+      #
+      # Note that if using RSpec, `TimeHelpers` are not included by default,
+      # and must be manually included by updating `rails_helper` accordingly:
+      #
+      # ```ruby
+      # RSpec.configure do |config|
+      #   config.include ActiveSupport::Testing::TimeHelpers
+      # end
+      # ```
+      #
+      # Moreover, because `TimeHelpers` relies on Minitest teardown hooks,
+      # `rails_helper` must be required (instead of `spec_helper`), or a
+      # similar adapter layer must be in effect.
+      #
+      # @example
+      #   # bad
+      #   Timecop
+      #
+      #   # bad
+      #   Timecop.freeze
+      #   Timecop.freeze(duration)
+      #   Timecop.freeze(time)
+      #
+      #   # good
+      #   freeze_time
+      #   travel(duration)
+      #   travel_to(time)
+      #
+      #   # bad
+      #   Timecop.freeze { assert true }
+      #   Timecop.freeze(duration) { assert true }
+      #   Timecop.freeze(time) { assert true }
+      #
+      #   # good
+      #   freeze_time { assert true }
+      #   travel(duration) { assert true }
+      #   travel_to(time) { assert true }
+      #
+      #   # bad
+      #   Timecop.travel(duration)
+      #   Timecop.travel(time)
+      #
+      #   # good
+      #   travel(duration)
+      #   travel_to(time)
+      #
+      #   # bad
+      #   Timecop.return
+      #   Timecop.return { assert true }
+      #
+      #   # good
+      #   travel_back
+      #   travel_to(time) { assert true }
+      #
+      #   # bad
+      #   Timecop.scale(factor)
+      #   Timecop.scale(factor) { assert true }
+      #
+      #   # good
+      #   travel(duration)
+      #   travel_to(time)
+      #   travel(duration) { assert true }
+      #   travel_to(time) { assert true }
+      class Timecop < Base
+        extend AutoCorrector
+
+        FREEZE_MESSAGE = 'Use `%<replacement>s` instead of `Timecop.freeze`'
+        FREEZE_WITH_ARGUMENTS_MESSAGE = 'Use `travel` or `travel_to` instead of `Timecop.freeze`'
+        RETURN_MESSAGE = 'Use `%<replacement>s` instead of `Timecop.return`'
+        FLOW_ADDENDUM = 'If you need time to keep flowing, simulate it by travelling again.'
+        TRAVEL_MESSAGE = "Use `travel` or `travel_to` instead of `Timecop.travel`. #{FLOW_ADDENDUM}"
+        SCALE_MESSAGE = "Use `travel` or `travel_to` instead of `Timecop.scale`. #{FLOW_ADDENDUM}"
+        MSG = 'Use `ActiveSupport::Testing::TimeHelpers` instead of `Timecop`'
+
+        def_node_matcher :timecop_const?, <<~PATTERN
+          (const {nil? cbase} :Timecop)
+        PATTERN
+
+        def_node_matcher :timecop_send, <<~PATTERN
+          (send
+            #timecop_const? ${:freeze :return :scale :travel}
+            $...
+          )
+        PATTERN
+
+        def on_const(node)
+          return unless timecop_const?(node)
+
+          timecop_send(node.parent) do |message, arguments|
+            return on_timecop_send(node.parent, message, arguments)
+          end
+
+          add_offense(node)
+        end
+
+        private
+
+        def on_timecop_send(node, message, arguments)
+          case message
+          when :freeze then on_timecop_freeze(node, arguments)
+          when :return then on_timecop_return(node, arguments)
+          when :scale  then on_timecop_scale(node, arguments)
+          when :travel then on_timecop_travel(node, arguments)
+          else add_offense(node)
+          end
+        end
+
+        def on_timecop_freeze(node, arguments)
+          if arguments.empty?
+            add_offense(node, message: format(FREEZE_MESSAGE, replacement: preferred_freeze_replacement)) do |corrector|
+              autocorrect_freeze(corrector, node, arguments)
+            end
+          else
+            add_offense(node, message: FREEZE_WITH_ARGUMENTS_MESSAGE)
+          end
+        end
+
+        def on_timecop_return(node, arguments)
+          add_offense(node, message: format(RETURN_MESSAGE, replacement: preferred_return_replacement)) do |corrector|
+            autocorrect_return(corrector, node, arguments)
+          end
+        end
+
+        def on_timecop_scale(node, _arguments)
+          add_offense(node, message: SCALE_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), preferred_freeze_replacement)
+        end
+
+        def autocorrect_return(corrector, node, _arguments)
+          return if given_block?(node)
+
+          corrector.replace(receiver_and_message_range(node), preferred_return_replacement)
+        end
+
+        def given_block?(node)
+          node.send_type? && node.parent && node.parent.block_type? && node.parent.send_node == node
+        end
+
+        def receiver_and_message_range(node)
+          node.source_range.with(end_pos: node.location.selector.end_pos)
+        end
+
+        def preferred_freeze_replacement
+          return 'travel_to(Time.now)' if target_rails_version < 5.2
+
+          'freeze_time'
+        end
+
+        def preferred_return_replacement
+          return 'travel_back' if target_rails_version < 6.0
+
+          'unfreeze_time'
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -122,6 +122,7 @@
 require_relative 'rails/to_s_with_argument'
 require_relative 'rails/top_level_hash_with_indifferent_access'
 require_relative 'rails/transaction_exit_statement'
+require_relative 'rails/timecop'
 require_relative 'rails/uniq_before_pluck'
 require_relative 'rails/unique_validation_without_index'
 require_relative 'rails/unknown_env'