[Fix rubocop#495] Add new Rails/I18nLocaleAssignment cop

Fixes rubocop#495

This PR adds new `Rails/I18nLocaleAssignment` cop.
It has a similar purpose to `Rails/TimeZoneAssignment` cop.
rubocop#458

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [#494](https://github.com/rubocop/rubocop-rails/pull/494): Add new `Rails/UnusedIgnoredColumns` cop. ([@pocke][])
 * [#490](https://github.com/rubocop/rubocop-rails/issues/490): Make `Rails/HttpStatus` aware of `head` method. ([@koic][])
 * [#483](https://github.com/rubocop/rubocop-rails/pull/483): Add new `Rails/EagerEvaluationLogMessage` cop. ([@aesthetikx][])
+* [#495](https://github.com/rubocop/rubocop-rails/issues/495): Add new `Rails/I18nLocaleAssignment` cop. ([@koic][])
 
 ### Bug fixes
 

config/default.yml

@@ -382,6 +382,14 @@ Rails/HttpStatus:
     - numeric
     - symbolic
 
+Rails/I18nLocaleAssignment:
+  Description: 'Prefer the usage of `I18n.with_locale` instead of manually updating `I18n.locale` value.'
+  Enabled: 'pending'
+  VersionAdded: '2.11'
+  Include:
+    - spec/**/*.rb
+    - test/**/*.rb
+
 Rails/IgnoredSkipActionFilterOption:
   Description: 'Checks that `if` and `only` (or `except`) are not used together as options of `skip_*` action filter.'
   Reference: 'https://api.rubyonrails.org/classes/AbstractController/Callbacks/ClassMethods.html#method-i-_normalize_callback_options'

docs/modules/ROOT/pages/cops.adoc

@@ -55,6 +55,7 @@ based on the https://rails.rubystyle.guide/[Rails Style Guide].
 * xref:cops_rails.adoc#railshelperinstancevariable[Rails/HelperInstanceVariable]
 * xref:cops_rails.adoc#railshttppositionalarguments[Rails/HttpPositionalArguments]
 * xref:cops_rails.adoc#railshttpstatus[Rails/HttpStatus]
+* xref:cops_rails.adoc#railsi18nlocaleassignment[Rails/I18nLocaleAssignment]
 * xref:cops_rails.adoc#railsignoredskipactionfilteroption[Rails/IgnoredSkipActionFilterOption]
 * xref:cops_rails.adoc#railsindexby[Rails/IndexBy]
 * xref:cops_rails.adoc#railsindexwith[Rails/IndexWith]

docs/modules/ROOT/pages/cops_rails.adoc

@@ -1976,6 +1976,47 @@ head 200
 | `numeric`, `symbolic`
 |===
 
+== Rails/I18nLocaleAssignment
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| No
+| 2.11
+| -
+|===
+
+This cop checks for the use of `I18n.locale=` method.
+
+The `locale` attribute persists for the rest of the Ruby runtime, potentially causing
+unexpected behavior at a later time.
+Using `I18n.with_locale` ensures the code passed in the block is the only place `I18n.locale` is affected.
+It eliminates the possibility of a `locale` sticking around longer than intended.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+I18n.locale = :fr
+
+# good
+I18n.with_locale(:fr) do
+end
+----
+
+=== Configurable attributes
+
+|===
+| Name | Default value | Configurable values
+
+| Include
+| `spec/**/*.rb`, `test/**/*.rb`
+| Array
+|===
+
 == Rails/IgnoredSkipActionFilterOption
 
 |===

lib/rubocop/cop/rails/i18n_locale_assignment.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # This cop checks for the use of `I18n.locale=` method.
+      #
+      # The `locale` attribute persists for the rest of the Ruby runtime, potentially causing
+      # unexpected behavior at a later time.
+      # Using `I18n.with_locale` ensures the code passed in the block is the only place `I18n.locale` is affected.
+      # It eliminates the possibility of a `locale` sticking around longer than intended.
+      #
+      # @example
+      #   # bad
+      #   I18n.locale = :fr
+      #
+      #   # good
+      #   I18n.with_locale(:fr) do
+      #   end
+      #
+      class I18nLocaleAssignment < Base
+        MSG = 'Use `I18n.with_locale` with block instead of `I18n.locale=`.'
+        RESTRICT_ON_SEND = %i[locale=].freeze
+
+        def_node_matcher :i18n_locale_assignment?, <<~PATTERN
+          (send (const {nil? cbase} :I18n) :locale= ...)
+        PATTERN
+
+        def on_send(node)
+          return unless i18n_locale_assignment?(node)
+
+          add_offense(node)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -44,6 +44,7 @@
 require_relative 'rails/helper_instance_variable'
 require_relative 'rails/http_positional_arguments'
 require_relative 'rails/http_status'
+require_relative 'rails/i18n_locale_assignment'
 require_relative 'rails/ignored_skip_action_filter_option'
 require_relative 'rails/index_by'
 require_relative 'rails/index_with'

spec/rubocop/cop/rails/i18n_locale_assignment_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::I18nLocaleAssignment, :config do
+  it 'registers an offense for `I18n.locale=`' do
+    expect_offense(<<~RUBY)
+      I18n.locale = :fr
+      ^^^^^^^^^^^^^^^^^ Use `I18n.with_locale` with block instead of `I18n.locale=`.
+    RUBY
+  end
+
+  it 'registers an offense for `::I18n.locale=`' do
+    expect_offense(<<~RUBY)
+      ::I18n.locale = :fr
+      ^^^^^^^^^^^^^^^^^^^ Use `I18n.with_locale` with block instead of `I18n.locale=`.
+    RUBY
+  end
+
+  it 'accepts `I18n.with_locale`' do
+    expect_no_offenses(<<~RUBY)
+      I18n.with_locale(:fr) do
+        do_something
+      end
+    RUBY
+  end
+end