Check commit rake task

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#7735](https://github.com/rubocop-hq/rubocop/issues/7735): `NodePattern` and `AST` classes have been moved to the [`rubocop-ast` gem](https://github.com/rubocop-hq/rubocop-ast). ([@marcandre][])
 * [#7950](https://github.com/rubocop-hq/rubocop/pull/7950): Add new `Lint/DeprecatedOpenSSLConstant` cop. ([@bdewater][])
 * [#7976](https://github.com/rubocop-hq/rubocop/issues/7976): Add `AllowAliasSyntax` and `AllowedMethods` options for `Layout/EmptyLinesAroundAttributeAccessor`. ([@koic][])
+* [#7984](https://github.com/rubocop-hq/rubocop/pull/7984): New `rake` task "check_commit" will run `rspec` and `rubocop` on files touched by the last commit. ([@marcandre][])
 
 ### Bug fixes
 

manual/contributing.md

@@ -17,6 +17,10 @@ Patches in any form are always welcome! GitHub pull requests are even better! :-
 Before submitting a patch or a pull request make sure all tests are
 passing and that your patch is in line with the [contribution
 guidelines](https://github.com/rubocop-hq/rubocop/blob/master/CONTRIBUTING.md).
+
+A handy way to test only the files that you have modified in the last commit
+(with `rspec` and `rubocop`) is to use `rake check_commit`.
+
 Also see the [Development section](development.md).
 
 ## Documentation

tasks/check_commit.rake

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+def commit_paths(commit_range)
+  commit_range = "#{commit_range}~..HEAD" if commit_range.include?('..')
+  `git diff-tree --no-commit-id --name-only -r #{commit_range}`.split("\n")
+ensure
+  exit($CHILD_STATUS.exitstatus) if $CHILD_STATUS.exitstatus != 0
+end
+
+desc 'Check files modified in commit (default: HEAD) with rspec and rubocop'
+RuboCop::RakeTask.new(:check_commit, :commit) do |t, args|
+  commit = args[:commit] || 'HEAD'
+  paths = commit_paths(commit)
+  specs = paths.select { |p| p.start_with?('spec') }
+
+  puts "Checking: #{paths.join(' ')}"
+  RuboCop::SpecRunner.new(specs).run_specs
+
+  t.patterns = paths
+end

tasks/spec_runner.rake

@@ -11,7 +11,10 @@ module RuboCop
   # The specs will be run in parallel if the system implements `fork`.
   # If ENV['COVERAGE'] is truthy, code coverage will be measured.
   class SpecRunner
-    def initialize(external_encoding: 'UTF-8', internal_encoding: nil)
+    attr_reader :rspec_args
+
+    def initialize(rspec_args = %w[spec], external_encoding: 'UTF-8', internal_encoding: nil)
+      @rspec_args = rspec_args
       @previous_external_encoding = Encoding.default_external
       @previous_internal_encoding = Encoding.default_internal
 
@@ -20,8 +23,6 @@ module RuboCop
     end
 
     def run_specs
-      rspec_args = %w[spec]
-
       n_failures = with_encoding do
         if Process.respond_to?(:fork)
           parallel_runner_klass.new(rspec_args).execute