[Fix rubocop#595] Exclude files ignored by git

CHANGELOG.md

@@ -10,6 +10,10 @@
 * [#6389](https://github.com/rubocop-hq/rubocop/pull/6389): Fix false negative for `Style/TrailingCommaInHashLitera`/`Style/TrailingCommaInArrayLiteral` when there is a comment in the last line. ([@bayandin][])
 * [#6566](https://github.com/rubocop-hq/rubocop/issues/6566): Fix false positive for `Layout/EmptyLinesAroundAccessModifier` when at the end of specifying a superclass is missing blank line. ([@koic][])
 
+### Changes
+
+* [#595](https://github.com/rubocop-hq/rubocop/issues/595): Exclude files ignored by `git`. ([@AlexWayfer][])
+
 ## 0.61.1 (2018-12-06)
 
 ### Bug fixes

lib/rubocop/target_finder.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'set'
+require 'open3'
 
 module RuboCop
   # This class finds target files to inspect by scanning the directory tree
@@ -59,10 +60,13 @@ def target_files_in_dir(base_dir = Dir.pwd)
       end
       all_files = find_files(base_dir, File::FNM_DOTMATCH)
       hidden_files = Set.new(all_files - find_files(base_dir, 0))
+
+      git_files = ls_git_files(base_dir)
+
       base_dir_config = @config_store.for(base_dir)
 
       target_files = all_files.select do |file|
-        to_inspect?(file, hidden_files, base_dir_config)
+        to_inspect?(file, git_files, hidden_files, base_dir_config)
       end
 
       # Most recently modified file first.
@@ -71,13 +75,6 @@ def target_files_in_dir(base_dir = Dir.pwd)
       target_files
     end
 
-    def to_inspect?(file, hidden_files, base_dir_config)
-      return false if base_dir_config.file_to_exclude?(file)
-      return true if !hidden_files.include?(file) && ruby_file?(file)
-
-      base_dir_config.file_to_include?(file)
-    end
-
     # Search for files recursively starting at the given base directory using
     # the given flags that determine how the match is made. Excluded files will
     # be removed later by the caller, but as an optimization find_files removes
@@ -100,6 +97,31 @@ def find_files(base_dir, flags)
       Dir.glob(pattern, flags).select { |path| FileTest.file?(path) }
     end
 
+    private
+
+    def ls_git_files(base_dir)
+      return if `sh -c 'command -v git'`.empty?
+
+      output, _error, status = Open3.capture3(
+        'git', 'ls-files', '-z', base_dir,
+        '--exclude-standard', '--others', '--cached', '--modified'
+      )
+
+      return unless status.success?
+
+      output.split("\0").map { |git_file| "#{base_dir}/#{git_file}" }
+    end
+
+    def to_inspect?(file, git_files, hidden_files, base_dir_config)
+      return false if base_dir_config.file_to_exclude?(file)
+
+      if !hidden_files.include?(file) && ruby_file?(file)
+        return git_files.nil? || git_files.include?(file)
+      end
+
+      base_dir_config.file_to_include?(file)
+    end
+
     def toplevel_dirs(base_dir, flags)
       Dir.glob(File.join(base_dir, '*'), flags).select do |dir|
         File.directory?(dir) && !dir.end_with?('/.', '/..')

manual/configuration.md

@@ -229,6 +229,7 @@ ruby interpreters listed under `AllCops`/`RubyInterpreters` are
 inspected, unless the file also matches a pattern in
 `AllCops`/`Exclude`. Hidden directories (i.e., directories whose names
 start with a dot) are not searched by default.
+Files ignored by Git are ignored by RuboCop by default.
 
 Here is an example that might be used for a Rails project:
 

spec/rubocop/target_finder_spec.rb

@@ -398,6 +398,65 @@
       expect(found_basenames).not_to include('ruby2.rb')
     end
 
+    context 'in git repo directory' do
+      before do
+        system 'git init'
+
+        create_empty_file('git_regular.rb')
+        create_empty_file('git_group.rb')
+        create_empty_file('git_excluded.rb')
+        create_empty_file('git_glob.rb')
+        create_empty_file('git with spaces.rb')
+        create_empty_file('git_dir/thing.rb')
+        create_file('.gitignore', <<-'CONTENT'.strip_indent)
+          git_regular.rb
+          git_gr[ao]up.rb
+          !git_excluded.rb
+          git_glob*
+          git_dir/
+        CONTENT
+      end
+
+      it 'does not pick files ignored by git' do
+        expect(found_basenames).not_to include(
+          'git_regular.rb',
+          'git_group.rb',
+          'git_glob.rb',
+          'git_dir/thing.rb'
+        )
+        expect(found_basenames).to include(
+          'git_excluded.rb',
+          'git with spaces.rb'
+        )
+      end
+
+      context 'without `git` executable' do
+        before do
+          allow_any_instance_of(Kernel).to receive(:`)
+            .with(/command -v git/)
+            .and_return('')
+
+          allow_any_instance_of(Kernel).to receive(:`)
+            .with(start_with('git ls-files'))
+            .and_raise(Errno::ENOENT, 'git')
+        end
+
+        it 'does not fail' do
+          expect { found_basenames }.not_to raise_error
+        end
+      end
+
+      context 'with `*` in argument' do
+        let(:base_dir) { './git*' }
+
+        it 'does not fail' do
+          expect(found_basenames).not_to include(
+            'thing.rb'
+          )
+        end
+      end
+    end
+
     context 'when an exception is raised while reading file' do
       around do |example|
         original_stderr = $stderr