[Fix #1442] Fix an incorrect behavior when using AllCops: TargetRailsVersion

This PR fixes an incorrect behavior when using `AllCops: TargetRailsVersion`.

This suppresses the following warning when `TargetRailsVersion` is defined on the user side:

```console
$ bundle exec rubocop
(snip)

Warning: AllCops does not support TargetRailsVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - DocumentationBaseURL
  - DocumentationExtension
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - NewCops
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion
  - ParserEngine
  - SuggestExtensions
  - ActiveSupportExtensionsEnabled
  - StringLiteralsFrozenByDefault
```

This is a dirty hack relying on a private constant to prevent the warning.
It should be updated to a better design in the future.

Fixes #1442.

Author: koic

changelog/fix_an_error_when_using_target_rails_version.md

@@ -0,0 +1 @@
+* [#1442](https://github.com/rubocop/rubocop-rails/issues/1442): Fix an incorrect behavior when using `AllCops: TargetRailsVersion`. ([@koic][])

lib/rubocop/rails/plugin.rb

@@ -24,8 +24,25 @@ def rules(_context)
 
         ConfigObsoletion.files << project_root.join('config', 'obsoletion.yml')
 
+        # FIXME: This is a dirty hack relying on a private constant to prevent
+        # "Warning: AllCops does not support TargetRailsVersion parameter".
+        # It should be updated to a better design in the future.
+        without_warnings do
+          ConfigValidator.const_set(:COMMON_PARAMS, ConfigValidator::COMMON_PARAMS.dup << 'TargetRailsVersion')
+        end
+
         LintRoller::Rules.new(type: :path, config_format: :rubocop, value: project_root.join('config', 'default.yml'))
       end
+
+      private
+
+      def without_warnings
+        original_verbose = $VERBOSE
+        $VERBOSE = nil
+        yield
+      ensure
+        $VERBOSE = original_verbose
+      end
     end
   end
 end