[Fix #8650] Improve hidden files finder performance (#8784)

Co-authored-by: Bozhidar Batsov <bozhidar@batsov.com>

CHANGELOG.md

@@ -12,7 +12,9 @@
 
 ### Changes
 
+* [#8785](https://github.com/rubocop-hq/rubocop/pull/8785): Update TargetRubyVersion 2.8 to 3.0 (experimental). ([@em-gazelle][])
 * [#8785](https://github.com/rubocop-hq/rubocop/pull/8785): Update TargetRubyVersion 2.8 to 3.0 (experimental). ([@koic][])
+* [#8650](https://github.com/rubocop-hq/rubocop/issues/8650): Faster find of hidden files in `TargetFinder` class which improves rubocop initial startup speed. ([@tleish][])
 
 ## 0.91.1 (2020-09-23)
 
@@ -4916,3 +4918,4 @@
 [@fsateler]: https://github.com/fsateler
 [@iSarCasm]: https://github.com/iSarCasm
 [@em-gazelle]: https://github.com/em-gazelle
+[@tleish]: https://github.com/tleish

lib/rubocop/target_finder.rb

@@ -5,6 +5,8 @@ module RuboCop
   # and picking ruby files.
   # @api private
   class TargetFinder
+    HIDDEN_PATH_SUBSTRING = "#{File::SEPARATOR}."
+
     def initialize(config_store, options = {})
       @config_store = config_store
       @options = options
@@ -55,7 +57,8 @@ def target_files_in_dir(base_dir = Dir.pwd)
       # Support Windows: Backslashes from command-line -> forward slashes
       base_dir = base_dir.gsub(File::ALT_SEPARATOR, File::SEPARATOR) if File::ALT_SEPARATOR
       all_files = find_files(base_dir, File::FNM_DOTMATCH)
-      hidden_files = Set.new(all_files - find_files(base_dir, 0))
+      # use file.include? for performance optimization
+      hidden_files = all_files.select { |file| file.include?(HIDDEN_PATH_SUBSTRING) }
       base_dir_config = @config_store.for(base_dir)
 
       target_files = all_files.select do |file|