Add new Rails/TimeUnitPluralization cop

This PR adds new `Rails/TimeUnitPluralization` cop.

It checks the usage of singular and plural method forms for
`year`, `week`, `month`, `day`, `hour`, `minute`, and `second`.

```ruby
# bad
1.hours
42.minute

# good
1.hour
42.minutes
```

changelog/new_add_new_rails_time_unit_pluralization_cop.md

@@ -0,0 +1 @@
+* [#1074](https://github.com/rubocop/rubocop-rails/pull/1074): Add new `Rails/TimeUnitPluralization` cop. ([@koic][])

config/default.yml

@@ -1039,6 +1039,11 @@ Rails/ThreeStateBooleanColumn:
   Include:
     - db/**/*.rb
 
+Rails/TimeUnitPluralization:
+  Description: 'Checks the usage of singular and plural method forms for time units.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Rails/TimeZone:
   Description: 'Checks the correct usage of time zone aware methods.'
   StyleGuide: 'https://rails.rubystyle.guide#time'

lib/rubocop/cop/rails/time_unit_pluralization.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks the usage of singular and plural method forms for
+      # `year`, `week`, `month`, `day`, `hour`, `minute`, and `second`.
+      #
+      # @example
+      #
+      #   # bad
+      #   1.hours
+      #   42.minute
+      #
+      #   # good
+      #   1.hour
+      #   42.minutes
+      #
+      class TimeUnitPluralization < Base
+        extend AutoCorrector
+
+        MSG = 'Use `%<preferred_method>s`.'
+
+        RESTRICT_ON_SEND = %i[
+          year years month months week weeks day days hour hours minute minutes second seconds
+        ].freeze
+
+        PLURALS = {
+          year: :years,
+          month: :months,
+          week: :weeks,
+          day: :days,
+          hour: :hours,
+          minute: :minutes,
+          second: :seconds
+        }.freeze
+
+        SINGULARS = PLURALS.invert.freeze
+
+        def on_send(node)
+          return unless node.receiver.int_type?
+
+          method_name = node.method_name
+          preferred_method = node.receiver.source == '1' ? SINGULARS[method_name] : PLURALS[method_name]
+          return unless preferred_method
+
+          range = node.loc.selector
+          message = format(MSG, preferred_method: preferred_method)
+
+          add_offense(range, message: message) do |corrector|
+            corrector.replace(range, preferred_method)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -116,6 +116,7 @@
 require_relative 'rails/strip_heredoc'
 require_relative 'rails/table_name_assignment'
 require_relative 'rails/three_state_boolean_column'
+require_relative 'rails/time_unit_pluralization'
 require_relative 'rails/time_zone'
 require_relative 'rails/time_zone_assignment'
 require_relative 'rails/to_formatted_s'

spec/rubocop/cop/rails/time_unit_pluralization_spec.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::TimeUnitPluralization, :config do
+  it 'registers an offense when using incorrect singulars' do
+    expect_offense(<<~RUBY)
+      1.years
+        ^^^^^ Use `year`.
+      1.months
+        ^^^^^^ Use `month`.
+      1.weeks
+        ^^^^^ Use `week`.
+      1.days
+        ^^^^ Use `day`.
+      1.hours
+        ^^^^^ Use `hour`.
+      1.minutes
+        ^^^^^^^ Use `minute`.
+      1.seconds
+        ^^^^^^^ Use `second`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      1.year
+      1.month
+      1.week
+      1.day
+      1.hour
+      1.minute
+      1.second
+    RUBY
+  end
+
+  it 'registers an offense when using incorrect plural form' do
+    expect_offense(<<~RUBY)
+      0.year
+        ^^^^ Use `years`.
+      0.month
+        ^^^^^ Use `months`.
+      0.week
+        ^^^^ Use `weeks`.
+      0.day
+        ^^^ Use `days`.
+      0.hour
+        ^^^^ Use `hours`.
+      0.minute
+        ^^^^^^ Use `minutes`.
+      0.second
+        ^^^^^^ Use `seconds`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      0.years
+      0.months
+      0.weeks
+      0.days
+      0.hours
+      0.minutes
+      0.seconds
+    RUBY
+  end
+
+  it 'does not register an offense when using correct singular form' do
+    expect_no_offenses(<<~RUBY)
+      1.hour
+    RUBY
+  end
+
+  it 'does not register an offense when using correct plural form' do
+    expect_no_offenses(<<~RUBY)
+      0.hours
+    RUBY
+  end
+
+  it 'does not register an offense when receiver is a variable' do
+    expect_no_offenses(<<~RUBY)
+      var.hour
+      var.hours
+    RUBY
+  end
+end