Add new Rails/ToFormattedS cop

Follow up rubocop/rails-style-guide#314.

This cop checks for consistent uses of `to_fs` or `to_formatted_s`,
depending on the cop's configuration.

## EnforcedStyle: to_fs (default)

```ruby
# bad
time.to_formatted_s(:db)

# good
time.to_fs(:db)
```

## EnforcedStyle: to_formatted_s

```
# bad
time.to_fs(:db)

# good
time.to_formatted_s(:db)
```

changelog/new_add_new_rails_to_formatted_s_cop.md

@@ -0,0 +1 @@
+* [#691](https://github.com/rubocop/rubocop-rails/pull/691): Add new `Rails/ToFormattedS` cop. ([@koic][])

config/default.yml

@@ -905,6 +905,15 @@ Rails/TimeZoneAssignment:
     - spec/**/*.rb
     - test/**/*.rb
 
+Rails/ToFormattedS:
+  Description: 'Checks for consistent uses of `to_fs` or `to_formatted_s`.'
+  Enabled: pending
+  EnforcedStyle: to_fs
+  SupportedStyles:
+    - to_fs
+    - to_formatted_s
+  VersionAdded: '<<next>>'
+
 Rails/TransactionExitStatement:
   Description: 'Avoid the usage of `return`, `break` and `throw` in transaction blocks.'
   Enabled: pending

lib/rubocop/cop/rails/to_formatted_s.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # This cop checks for consistent uses of `to_fs` or `to_formatted_s`,
+      # depending on the cop's configuration.
+      #
+      # @example EnforcedStyle: to_fs (default)
+      #
+      #   # bad
+      #   time.to_formatted_s(:db)
+      #
+      #   # good
+      #   time.to_fs(:db)
+      #
+      # @example EnforcedStyle: to_formatted_s
+      #
+      #   # bad
+      #   time.to_fs(:db)
+      #
+      #   # good
+      #   time.to_formatted_s(:db)
+      #
+      class ToFormattedS < Base
+        include ConfigurableEnforcedStyle
+        extend AutoCorrector
+        extend TargetRailsVersion
+
+        minimum_target_rails_version 7.0
+
+        MSG = 'Use `%<prefer>s` instead.'
+        RESTRICT_ON_SEND = %i[to_formatted_s to_fs].freeze
+
+        def on_send(node)
+          return if node.method?(style)
+
+          add_offense(node.loc.selector, message: format(MSG, prefer: style)) do |corrector|
+            corrector.replace(node.loc.selector, style)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -107,6 +107,7 @@
 require_relative 'rails/table_name_assignment'
 require_relative 'rails/time_zone'
 require_relative 'rails/time_zone_assignment'
+require_relative 'rails/to_formatted_s'
 require_relative 'rails/transaction_exit_statement'
 require_relative 'rails/uniq_before_pluck'
 require_relative 'rails/unique_validation_without_index'

spec/rubocop/cop/rails/to_formatted_s_spec.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ToFormattedS, :config do
+  context 'Rails >= 7.0', :rails70 do
+    context 'EnforcedStyle: to_fs' do
+      let(:cop_config) { { 'EnforcedStyle' => 'to_fs' } }
+
+      it 'registers and corrects an offense when using `to_formatted_s`' do
+        expect_offense(<<~RUBY)
+          time.to_formatted_s(:db)
+               ^^^^^^^^^^^^^^ Use `to_fs` instead.
+        RUBY
+
+        expect_correction(<<~RUBY)
+          time.to_fs(:db)
+        RUBY
+      end
+
+      it 'does not register an offense when using `to_fs`' do
+        expect_no_offenses(<<~RUBY)
+          time.to_fs(:db)
+        RUBY
+      end
+    end
+
+    context 'EnforcedStyle: to_formatted_s' do
+      let(:cop_config) { { 'EnforcedStyle' => 'to_formatted_s' } }
+
+      it 'registers and corrects an offense when using `formatted_s`' do
+        expect_offense(<<~RUBY)
+          time.to_fs(:db)
+               ^^^^^ Use `to_formatted_s` instead.
+        RUBY
+
+        expect_correction(<<~RUBY)
+          time.to_formatted_s(:db)
+        RUBY
+      end
+
+      it 'does not register an offense when using `to_formatted_s`' do
+        expect_no_offenses(<<~RUBY)
+          time.to_formatted_s(:db)
+        RUBY
+      end
+    end
+  end
+
+  context 'Rails <= 6.1', :rails61 do
+    context 'EnforcedStyle: to_fs' do
+      let(:cop_config) { { 'EnforcedStyle' => 'to_fs' } }
+
+      it 'does not register an offense when using `to_formatted_s`' do
+        expect_no_offenses(<<~RUBY)
+          time.to_formatted_s(:db)
+        RUBY
+      end
+    end
+
+    context 'EnforcedStyle: to_formatted_s' do
+      let(:cop_config) { { 'EnforcedStyle' => 'to_formatted_s' } }
+
+      it 'does not register an offense when using `formatted_s`' do
+        expect_no_offenses(<<~RUBY)
+          time.to_fs(:db)
+        RUBY
+      end
+    end
+  end
+end

spec/support/shared_contexts.rb

@@ -23,3 +23,7 @@
 RSpec.shared_context 'with Rails 6.1', :rails61 do
   let(:rails_version) { 6.1 }
 end
+
+RSpec.shared_context 'with Rails 7.0', :rails70 do
+  let(:rails_version) { 7.0 }
+end