find possibly unused modules in a rails app

find_unused_modules.rb

code = Dir['{app,config,lib}/**/*.{rb,slim,erb,rake}'].map { File.read _1 }.join; 1
defs = code.scan(/(?:class|module) \K[A-Z][\w:]+(?=\n)/); 1
refs = code.scan(/(?<!class |module )\b#{Regexp.union(defs + defs.map { |d| d.split('::').last })}\b/); 1
unused = defs.uniq.sort.select { |d| refs.count(d) == 0 && refs.count(d.split('::').last) == 0 }