which_class_defines.rb

# Classes are from https://docs.ruby-lang.org/ja/2.6.0/library/_builtin.html
classes = [
  BasicObject,
  Object,
  Array,
  Binding,
  ConditionVariable,
  Data,
  Dir,
  Encoding,
  Enumerator,
  FalseClass,
  Fiber,
  Hash,
  IO,
  File,
  MatchData,
  Method,
  Module,
  Class,
  Mutex,
  NilClass,
  Numeric,
  Bignum,
  Complex,
  Fixnum,
  Float,
  Integer,
  Rational,
  Proc,
  Queue,
  Random,
  Range,
  Regexp,
  RubyVM,
  String,
  Struct,
  Symbol,
  Thread,
  SizedQueue,
  ThreadGroup,
  Time,
  TracePoint,
  TrueClass,
  UnboundMethod,
]

searchee_method = ARGV[0]

classes_defines_this = []
classes.each do |c|
  methods = [c.methods(false), c.instance_methods(false)].flatten.map(&:to_s)
  classes_defines_this << c if methods.include?(searchee_method)
end

if classes_defines_this.count > 0
  puts "The method \"#{searchee_method}\" is defined at #{classes_defines_this.join(", ")}"
else
  puts "No class defines the method \"#{searchee_method}\"."
  exit 1
end