[Fix rubocop#704] Add new Rails/StripHeredoc cop

Closes rubocop#704.

Enforces the use of squiggly heredoc over `strip_heredoc`.

```ruby
# bad
<<EOS.strip_heredoc
  some text
EOS

# bad
<<-EOS.strip_heredoc
  some text
EOS

# good
<<~EOS
  some text
EOS
```

changelog/new_add_new_rails_strip_heredoc_cop.md

@@ -0,0 +1 @@
+* [#704](https://github.com/rubocop/rubocop-rails/issues/704): Add new `Rails/StripHeredoc` cop. ([@koic][])

config/default.yml

@@ -871,6 +871,11 @@ Rails/SquishedSQLHeredocs:
   # to be preserved in order to work, thus autocorrection is not safe.
   SafeAutoCorrect: false
 
+Rails/StripHeredoc:
+  Description: 'Enforces the use of squiggly heredoc over `strip_heredoc`.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Rails/TableNameAssignment:
   Description: >-
     Do not use `self.table_name =`. Use Inflections or `table_name_prefix` instead.

lib/rubocop/cop/rails/strip_heredoc.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Enforces the use of squiggly heredoc over `strip_heredoc`.
+      #
+      # @example
+      #
+      #   # bad
+      #   <<EOS.strip_heredoc
+      #     some text
+      #   EOS
+      #
+      #   # bad
+      #   <<-EOS.strip_heredoc
+      #     some text
+      #   EOS
+      #
+      #   # good
+      #   <<~EOS
+      #     some text
+      #   EOS
+      #
+      class StripHeredoc < Base
+        extend AutoCorrector
+        extend TargetRubyVersion
+
+        MSG = 'Use squiggly heredoc (`<<~`) instead of `strip_heredoc`.'
+
+        minimum_target_ruby_version 2.3
+
+        def on_send(node)
+          return unless node.method?(:strip_heredoc)
+          return unless (receiver = node.receiver)
+          return unless receiver.str_type? || receiver.dstr_type?
+          return unless receiver.respond_to?(:heredoc?) && receiver.heredoc?
+
+          register_offense(node, receiver)
+        end
+
+        private
+
+        def register_offense(node, heredoc_identifier)
+          add_offense(node) do |corrector|
+            replacement = heredoc_identifier.source.gsub(/\A<<-?/, '<<~')
+
+            corrector.replace(heredoc_identifier, replacement)
+            corrector.remove(node.loc.dot)
+            corrector.remove(node.loc.selector)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -104,6 +104,7 @@
 require_relative 'rails/short_i18n'
 require_relative 'rails/skips_model_validations'
 require_relative 'rails/squished_sql_heredocs'
+require_relative 'rails/strip_heredoc'
 require_relative 'rails/table_name_assignment'
 require_relative 'rails/time_zone'
 require_relative 'rails/time_zone_assignment'

spec/rubocop/cop/rails/strip_heredoc_spec.rb

@@ -0,0 +1,99 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::StripHeredoc, :config do
+  context 'Ruby <= 2.2', :ruby22 do
+    it 'does not register an offense when using `strip_heredoc`' do
+      expect_no_offenses(<<~RUBY)
+        <<-EOS.strip_heredoc
+          some text
+        EOS
+      RUBY
+    end
+  end
+
+  context 'Ruby >= 2.3', :ruby23 do
+    it 'registers an offense when using `strip_heredoc` with `<<`' do
+      expect_offense(<<~RUBY)
+        <<EOS.strip_heredoc
+        ^^^^^^^^^^^^^^^^^^^ Use squiggly heredoc (`<<~`) instead of `strip_heredoc`.
+          some text
+        EOS
+      RUBY
+
+      expect_correction(<<~RUBY)
+        <<~EOS
+          some text
+        EOS
+      RUBY
+    end
+
+    it 'registers an offense when using `strip_heredoc` with `<<-`' do
+      expect_offense(<<~RUBY)
+        <<-EOS.strip_heredoc
+        ^^^^^^^^^^^^^^^^^^^^ Use squiggly heredoc (`<<~`) instead of `strip_heredoc`.
+          some text
+        EOS
+      RUBY
+
+      expect_correction(<<~RUBY)
+        <<~EOS
+          some text
+        EOS
+      RUBY
+    end
+
+    it 'registers an offense when using `strip_heredoc.do_something`' do
+      expect_offense(<<~RUBY)
+        <<-EOS.strip_heredoc.do_something
+        ^^^^^^^^^^^^^^^^^^^^ Use squiggly heredoc (`<<~`) instead of `strip_heredoc`.
+          some text
+        EOS
+      RUBY
+
+      expect_correction(<<~RUBY)
+        <<~EOS.do_something
+          some text
+        EOS
+      RUBY
+    end
+
+    it 'registers an offense when using `strip_heredoc` with multiline text' do
+      expect_offense(<<~RUBY)
+        <<-EOS.strip_heredoc
+        ^^^^^^^^^^^^^^^^^^^^ Use squiggly heredoc (`<<~`) instead of `strip_heredoc`.
+          some text
+          some text
+        EOS
+      RUBY
+
+      expect_correction(<<~RUBY)
+        <<~EOS
+          some text
+          some text
+        EOS
+      RUBY
+    end
+
+    it 'does not register an offense when using squiggly heredoc' do
+      expect_no_offenses(<<~RUBY)
+        <<~EOS
+          some text
+        EOS
+      RUBY
+    end
+
+    it 'does not register an offense when using `do_something.strip_heredoc`' do
+      expect_no_offenses(<<~RUBY)
+        <<-EOS.do_something.strip_heredoc
+          some text
+        EOS
+      RUBY
+    end
+
+    it 'does not register an offense when using `strip_heredoc` without receiver' do
+      expect_no_offenses(<<~RUBY)
+        strip_heredoc
+      RUBY
+    end
+  end
+end