A Rubocop wrapper that checks only added/modified code

rubocop.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

# Source: https://gist.github.com/skanev/9d4bec97d5a6825eaaf6
#
# A sneaky wrapper around Rubocop that allows you to run it only against
# the recent changes, as opposed to the whole project. It lets you
# enforce the style guide for new/modified code only, as opposed to
# having to restyle everything or adding cops incrementally. It relies
# on git to figure out which files to check.
#
# Here are some options you can pass in addition to the ones in rubocop:
#
#   --local               Check only the changes you are about to push
#                         to the remote repository.
#
#   --uncommitted         Check only changes in files that have not been
#   --index               committed (i.e. either in working directory or
#                         staged).
#
#   --against REFSPEC     Check changes since REFSPEC. This can be
#                         anything that git will recognize.
#
#   --branch BASE_BRANCH  Check only changes in the current branch
#                         compared to the given BASE_BRANCH or `master`
#                         by default.
#
#   --courage             Without this option, only the modified lines
#                         are inspected. When supplied, it will check
#                         the full contents of the file. You should have
#                         the courage to fix your style violations as
#                         you see them, you know.
#
# Caveat emptor:
#
# * Monkey patching ahead. This script relies on Rubocop internals and
#   has been tested against 0.25.0. Newer (or older) versions might
#   break it.
#
# * While it does try to check modified lines only, there might be some
#   quirks. It might not show offenses in modified code if they are
#   reported at unmodified lines. It might also show offenses in
#   unmodified code if they are reported in modified lines.

require 'rubocop'

module DirtyCop
  extend self # In your face, style guide!

  def bury_evidence?(file, line)
    !report_offense_at?(file, line)
  end

  def uncovered_targets
    @files
  end

  def cover_up_unmodified(ref, only_changed_lines = true)
    @files = files_modified_since(ref)
    @line_filter = build_line_filter(@files, ref) if only_changed_lines
  end

  # rubocop:disable Metrics/CyclomaticComplexity
  def process_bribe
    eat_a_donut if ARGV.empty?

    ref = nil
    only_changed_lines = true

    loop do
      arg = ARGV.shift
      case arg
      when '--local'
        ref = `git rev-parse --abbrev-ref --symbolic-full-name @{u}`.chomp
        exit 1 unless $CHILD_STATUS.success?
      when '--against'
        ref = ARGV.shift
      when '--uncommitted', '--index'
        ref = 'HEAD'
      when '--branch'
        branch = ARGV.shift
        ref = `git merge-base HEAD #{branch || 'master'}`.chomp
      when '--courage'
        only_changed_lines = false
      else
        ARGV.unshift arg
        break
      end
    end

    return unless ref

    cover_up_unmodified ref, only_changed_lines
  end
  # rubocop:enable Metrics/CyclomaticComplexity

  private

  def report_offense_at?(file, line)
    !@line_filter || @line_filter.fetch(file)[line]
  end

  def files_modified_since(ref)
    `git diff --diff-filter=AM --name-only #{ref}`.
      lines.
      map(&:chomp).
      grep(/\.rb$/).
      map { |file| File.absolute_path(file) }
  end

  def build_line_filter(_files, ref)
    result = {}

    suspects = files_modified_since(ref)
    suspects.each do |file|
      result[file] = lines_modified_since(file, ref)
    end

    result
  end

  def lines_modified_since(file, ref)
    ranges =
      `git diff -p -U0 #{ref} #{file}`.
        lines.
        grep(/^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@/) { $1.to_i..($1.to_i + $2.to_i) }. # rubocop:disable Style/PerlBackrefs
        reverse

    mask = Array.new(ranges.first.end)

    ranges.each do |range|
      range.each do |line|
        mask[line] = true
      end
    end

    mask
  end

  def eat_a_donut
    puts "#{$PROGRAM_NAME}: The dirty cop Alex Murphy could have been"
    puts
    puts File.read(__FILE__)[/(?:^#(?:[^!].*)?\n)+/s].gsub(/^#/, '   ')
    exit
  end
end

module RuboCop
  class TargetFinder
    alias find_unpatched find

    def find(args)
      targets = find_unpatched(args)
      replacements = DirtyCop.uncovered_targets
      return targets unless replacements

      replacements & targets
    end
  end

  class Runner
    alias file_offenses_unpatched file_offenses

    def file_offenses(file)
      file_offenses_unpatched(file).
        reject { |o| DirtyCop.bury_evidence?(o.location.source_buffer.name, o.location.line) }
    end
  end
end

DirtyCop.process_bribe

exit RuboCop::CLI.new.run

Made some fixes, tested on Rubocop 0.74.0.
Made it respect exclusions defined in the config.

Works great