ruby--ruby/doc/NEWS-2.7.0

# -*- rdoc -*-

= NEWS for Ruby 2.7.0

This document is a list of user visible feature changes made between
releases except for bug fixes.

Note that each entry is kept so brief that no reason behind or reference
information is supplied with.  For a full list of changes with all
sufficient information, see the ChangeLog file or Redmine
(e.g. <tt>https://bugs.ruby-lang.org/issues/$FEATURE_OR_BUG_NUMBER</tt>).

== Changes since the 2.6.0 release

=== Language changes

==== Pattern matching

* Pattern matching is introduced as an experimental feature. [Feature #14912]

    case [0, [1, 2, 3]]
    in [a, [b, *c]]
      p a #=> 0
      p b #=> 1
      p c #=> [2, 3]
    end

    case {a: 0, b: 1}
    in {a: 0, x: 1}
      :unreachable
    in {a: 0, b: var}
      p var #=> 1
    end

    case -1
    in 0 then :unreachable
    in 1 then :unreachable
    end #=> NoMatchingPatternError

    json = <<END
    {
      "name": "Alice",
      "age": 30,
      "children": [{ "name": "Bob", "age": 2 }]
    }
    END

    JSON.parse(json, symbolize_names: true) in {name: "Alice", children: [{name: name, age: age}]}

    p name #=> "Bob"
    p age  #=> 2

    JSON.parse(json, symbolize_names: true) in {name: "Alice", children: [{name: "Charlie", age: age}]}
    #=> NoMatchingPatternError

* See the following slides for more details:
  * https://speakerdeck.com/k_tsj/pattern-matching-new-feature-in-ruby-2-dot-7
  * Note that the slides are slightly obsolete.

* The warning against pattern matching can be suppressed with
  {-W:no-experimental option}[#label-Warning+option].

==== The spec of keyword arguments is changed towards 3.0

* Automatic conversion of keyword arguments and positional arguments is
  deprecated, and conversion will be removed in Ruby 3.  [Feature #14183]

  * When a method call passes a Hash at the last argument, and when it
    passes no keywords, and when the called method accepts keywords,
    a warning is emitted.  To continue treating the hash as keywords,
    add a double splat operator to avoid the warning and ensure
    correct behavior in Ruby 3.

      def foo(key: 42); end; foo({key: 42})   # warned
      def foo(**kw);    end; foo({key: 42})   # warned
      def foo(key: 42); end; foo(**{key: 42}) # OK
      def foo(**kw);    end; foo(**{key: 42}) # OK

  * When a method call passes keywords to a method that accepts keywords,
    but it does not pass enough required positional arguments, the
    keywords are treated as a final required positional argument, and a
    warning is emitted.  Pass the argument as a hash instead of keywords
    to avoid the warning and ensure correct behavior in Ruby 3.

      def foo(h, **kw); end; foo(key: 42)      # warned
      def foo(h, key: 42); end; foo(key: 42)   # warned
      def foo(h, **kw); end; foo({key: 42})    # OK
      def foo(h, key: 42); end; foo({key: 42}) # OK

  * When a method accepts specific keywords but not a keyword splat, and
    a hash or keywords splat is passed to the method that includes both
    Symbol and non-Symbol keys, the hash will continue to be split, and
    a warning will be emitted.  You will need to update the calling code
    to pass separate hashes to ensure correct behavior in Ruby 3.

      def foo(h={}, key: 42); end; foo("key" => 43, key: 42)   # warned
      def foo(h={}, key: 42); end; foo({"key" => 43, key: 42}) # warned
      def foo(h={}, key: 42); end; foo({"key" => 43}, key: 42) # OK

  * If a method does not accept keywords, and is called with keywords,
    the keywords are still treated as a positional hash, with no warning.
    This behavior will continue to work in Ruby 3.

      def foo(opt={});  end; foo( key: 42 )   # OK

* Non-symbols are allowed as keyword argument keys if the method accepts
  arbitrary keywords. [Feature #14183]

  * Non-Symbol keys in a keyword arguments hash were prohibited in 2.6.0,
    but are now allowed again.  [Bug #15658]

      def foo(**kw); p kw; end; foo("str" => 1) #=> {"str"=>1}

* <code>**nil</code> is allowed in method definitions to explicitly mark
  that the method accepts no keywords. Calling such a method with keywords
  will result in an ArgumentError. [Feature #14183]

    def foo(h, **nil); end; foo(key: 1)       # ArgumentError
    def foo(h, **nil); end; foo(**{key: 1})   # ArgumentError
    def foo(h, **nil); end; foo("str" => 1)   # ArgumentError
    def foo(h, **nil); end; foo({key: 1})     # OK
    def foo(h, **nil); end; foo({"str" => 1}) # OK

* Passing an empty keyword splat to a method that does not accept keywords
  no longer passes an empty hash, unless the empty hash is necessary for
  a required parameter, in which case a warning will be emitted.  Remove
  the double splat to continue passing a positional hash.  [Feature #14183]

    h = {}; def foo(*a) a end; foo(**h) # []
    h = {}; def foo(a) a end; foo(**h)  # {} and warning
    h = {}; def foo(*a) a end; foo(h)   # [{}]
    h = {}; def foo(a) a end; foo(h)    # {}

* Above warnings can be suppressed also with {-W:no-deprecated option}[#label-Warning+option].

==== Numbered parameters

* Numbered parameters as default block parameters are introduced. [Feature #4475]

    [1, 2, 10].map { _1.to_s(16) }    #=> ["1", "2", "a"]
    [[1, 2], [3, 4]].map { _1 + _2 }  #=> [3, 7]

  You can still define a local variable named +_1+ and so on,
  and that is honored when present, but renders a warning.

    _1 = 0            #=> warning: `_1' is reserved for numbered parameter; consider another name
    [1].each { p _1 } # prints 0 instead of 1

==== proc/lambda without block is deprecated

* Proc.new and Kernel#proc with no block in a method called with a block will
  now display a warning.

    def foo
      proc
    end
    foo { puts "Hello" } #=> warning: Capturing the given block using Kernel#proc is deprecated; use `&block` instead

  This warning can be suppressed with {-W:no-deprecated option}[#label-Warning+option].

* Kernel#lambda with no block in a method called with a block raises an exception.

    def bar
      lambda
    end
    bar { puts "Hello" } #=> tried to create Proc object without a block (ArgumentError)

==== Other miscellaneous changes

* A beginless range is experimentally introduced.  It might be useful
  in +case+, new call-sequence of the <code>Comparable#clamp</code>,
  constants and DSLs.  [Feature #14799]

     ary[..3]  # identical to ary[0..3]

     case RUBY_VERSION
     when ..."2.4" then puts "EOL"
     # ...
     end

     age.clamp(..100)

     where(sales: ..100)

* Setting <code>$;</code> to a non-nil value will now display a warning. [Feature #14240]
  This includes the usage in String#split.
  This warning can be suppressed with {-W:no-deprecated option}[#label-Warning+option].

* Setting <code>$,</code> to a non-nil value will now display a warning. [Feature #14240]
  This includes the usage in Array#join.
  This warning can be suppressed with {-W:no-deprecated option}[#label-Warning+option].

* Quoted here-document identifiers must end within the same line.

     <<"EOS
     " # This had been warned since 2.4; Now it raises a SyntaxError
     EOS

* The flip-flop syntax deprecation is reverted. [Feature #5400]

* Comment lines can be placed between fluent dot now.

    foo
      # .bar
      .baz # => foo.baz

* Calling a private method with a literal +self+ as the receiver
  is now allowed. [Feature #11297] [Feature #16123]

* Modifier rescue now operates the same for multiple assignment as single
  assignment. [Bug #8279]

    a, b = raise rescue [1, 2]
    # Previously parsed as: (a, b = raise) rescue [1, 2]
    # Now parsed as:         a, b = (raise rescue [1, 2])

* +yield+ in singleton class syntax will now display a warning. This behavior
  will soon be deprecated. [Feature #15575].

   def foo
     class << Object.new
       yield #=> warning: `yield' in class syntax will not be supported from Ruby 3.0. [Feature #15575]
     end
   end
   foo { p :ok }

  This warning can be suppressed with {-W:no-deprecated option}[#label-Warning+option].

* Argument forwarding by <code>(...)</code> is introduced. [Feature #16253]

    def foo(...)
      bar(...)
    end

  All arguments to +foo+ are forwarded to +bar+, including keyword and
  block arguments.
  Note that the parentheses are mandatory.  <code>bar ...</code> is parsed
  as an endless range.

* Access and setting of <code>$SAFE</code> will now always display a warning.
  <code>$SAFE</code> will become a normal global variable in Ruby 3.0.  [Feature #16131]

* <code>Object#{taint,untaint,trust,untrust}</code> and related functions in the C-API
  no longer have an effect (all objects are always considered untainted), and will now
  display a warning in verbose mode. This warning will be disabled even in non-verbose mode in
  Ruby 3.0, and the methods and C functions will be removed in Ruby 3.2. [Feature #16131]

* Refinements take place at Object#method and Module#instance_method. [Feature #15373]

=== Command line options

==== Warning option

The +-W+ option has been extended with a following +:+, to manage categorized
warnings.  [Feature #16345] [Feature #16420]

* To suppress deprecation warnings:

    $ ruby -e '$; = ""'
    -e:1: warning: `$;' is deprecated

    $ ruby -W:no-deprecated -e '$; = //'

* It works with the +RUBYOPT+ environment variable:

    $ RUBYOPT=-W:no-deprecated ruby -e '$; = //'

* To suppress experimental feature warnings:

    $ ruby -e '0 in a'
    -e:1: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!

    $ ruby -W:no-experimental -e '0 in a'

* To suppress both by using +RUBYOPT+, set space separated values:

    $ RUBYOPT='-W:no-deprecated -W:no-experimental' ruby -e '($; = "") in a'

See also Warning in {Core classes updates}[#label-Core+classes+updates+-28outstanding+ones+only-29].

=== Core classes updates (outstanding ones only)

[Array]

  [New methods]

    * Added Array#intersection. [Feature #16155]

    * Added Array#minmax, with a faster implementation than Enumerable#minmax. [Bug #15929]

[Comparable]

  [Modified method]

    * Comparable#clamp now accepts a Range argument. [Feature #14784]

        -1.clamp(0..2) #=> 0
         1.clamp(0..2) #=> 1
         3.clamp(0..2) #=> 2
        # With beginless and endless ranges:
        -1.clamp(0..)  #=> 0
         3.clamp(..2)  #=> 2


[Complex]

  [New method]

    * Added Complex#<=>.
      So <code>0 <=> 0i</code> will not raise NoMethodError. [Bug #15857]

[Dir]

  [Modified methods]

    * Dir.glob and Dir.[] no longer allow NUL-separated glob pattern.
      Use Array instead.  [Feature #14643]

[Encoding]

  [New encoding]

    * Added new encoding CESU-8. [Feature #15931]

[Enumerable]

  [New methods]

    * Added Enumerable#filter_map.  [Feature #15323]

        [1, 2, 3].filter_map {|x| x.odd? ? x.to_s : nil } #=> ["1", "3"]

    * Added Enumerable#tally.  [Feature #11076]

        ["A", "B", "C", "B", "A"].tally #=> {"A"=>2, "B"=>2, "C"=>1}

[Enumerator]

  [New methods]

    * Added Enumerator.produce to generate an Enumerator from any custom
      data transformation.  [Feature #14781]

        require "date"
        dates = Enumerator.produce(Date.today, &:succ) #=> infinite sequence of dates
        dates.detect(&:tuesday?) #=> next Tuesday

    * Added Enumerator::Lazy#eager that generates a non-lazy enumerator
      from a lazy enumerator.  [Feature #15901]

        a = %w(foo bar baz)
        e = a.lazy.map {|x| x.upcase }.map {|x| x + "!" }.eager
        p e.class               #=> Enumerator
        p e.map {|x| x + "?" }  #=> ["FOO!?", "BAR!?", "BAZ!?"]

    * Added Enumerator::Yielder#to_proc so that a Yielder object
      can be directly passed to another method as a block
      argument.  [Feature #15618]

[Fiber]

  [New method]

    * Added Fiber#raise that behaves like Fiber#resume but raises an
      exception on the resumed fiber.  [Feature #10344]

[File]

  [New method]

    * Added File.absolute_path? to check whether a path is absolute or
      not in a portable way. [Feature #15868]

        File.absolute_path?("/foo")   # => true (on *nix)
        File.absolute_path?("C:/foo") # => true (on Windows)
        File.absolute_path?("foo")    # => false

  [Modified method]

    * File.extname now returns a dot string for names ending with a dot on
      non-Windows platforms.  [Bug #15267]

          File.extname("foo.") #=> "."

[FrozenError]

  [New method]

    * Added FrozenError#receiver to return the frozen object on which
      modification was attempted.  To set this object when raising
      FrozenError in Ruby code, FrozenError.new accepts a +:receiver+
      option.  [Feature #15751]

[GC]

  [New method]

    * Added GC.compact method for compacting the heap.
      This function compacts live objects in the heap so that fewer pages may
      be used, and the heap may be more CoW (copy-on-write) friendly. [Feature #15626]

      Details on the algorithm and caveats can be found here:
      https://bugs.ruby-lang.org/issues/15626

[IO]

  [New method]

    * Added IO#set_encoding_by_bom to check the BOM and set the external
      encoding.  [Bug #15210]

[Integer]

  [Modified method]

    * Integer#[] now supports range operations.  [Feature #8842]

         0b01001101[2, 4]  #=> 0b0011
         0b01001100[2..5]  #=> 0b0011
         0b01001100[2...6] #=> 0b0011
         #   ^^^^

[Method]

  [Modified method]

    * Method#inspect shows more information. [Feature #14145]

[Module]

  [New methods]

    * Added Module#const_source_location to retrieve the location where a
      constant is defined.  [Feature #10771]

    * Added Module#ruby2_keywords for marking a method as passing keyword
      arguments through a regular argument splat, useful when delegating
      all arguments to another method in a way that can be backwards
      compatible with older Ruby versions.  [Bug #16154]

  [Modified methods]

    * Module#autoload? now takes an +inherit+ optional argument, like
      Module#const_defined?.  [Feature #15777]

    * Module#name now always returns a frozen String. The returned String is
      always the same for a given Module. This change is
      experimental. [Feature #16150]

[NilClass / TrueClass / FalseClass]

  [Modified methods]

    * NilClass#to_s, TrueClass#to_s, and FalseClass#to_s now always return a
      frozen String. The returned String is always the same for each of these
      values. This change is experimental. [Feature #16150]

[ObjectSpace::WeakMap]

  [Modified method]

    * ObjectSpace::WeakMap#[]= now accepts special objects as either key or
      values.  [Feature #16035]

[Proc]

  [New method]

    * Added Proc#ruby2_keywords for marking the proc as passing keyword
      arguments through a regular argument splat, useful when delegating
      all arguments to another method or proc in a way that can be backwards
      compatible with older Ruby versions.  [Feature #16404]

[Range]

  [New method]

    * Added Range#minmax, with a faster implementation than Enumerable#minmax.
      It returns a maximum that now corresponds to Range#max. [Bug #15807]

  [Modified method]

    * Range#=== now uses Range#cover? for String arguments, too (in Ruby 2.6, it was
      changed from Range#include? for all types except strings). [Bug #15449]


[RubyVM]

  [Removed method]

    * +RubyVM.resolve_feature_path+ moved to
      <code>$LOAD_PATH.resolve_feature_path</code>.  [Feature #15903] [Feature #15230]

[String]

  [Unicode]

    * Update Unicode version and Emoji version from 11.0.0 to
      12.0.0.  [Feature #15321]

    * Update Unicode version to 12.1.0, adding support for
      U+32FF SQUARE ERA NAME REIWA.  [Feature #15195]

    * Update Unicode Emoji version to 12.1. [Feature #16272]

[Symbol]

  [New methods]

    * Added Symbol#start_with? and Symbol#end_with? methods.  [Feature #16348]

[Time]

  [New methods]

    * Added Time#ceil method.  [Feature #15772]

    * Added Time#floor method.  [Feature #15653]

  [Modified method]

    * Time#inspect is separated from Time#to_s and it shows
      the time's sub second.  [Feature #15958]

[UnboundMethod]

  [New method]

    * Added UnboundMethod#bind_call method.  [Feature #15955]

      <code>umethod.bind_call(obj, ...)</code> is semantically equivalent
      to <code>umethod.bind(obj).call(...)</code>.  This idiom is used in
      some libraries to call a method that is overridden.  The added
      method does the same without allocation of an intermediate Method
      object.

          class Foo
            def add_1(x)
              x + 1
            end
          end
          class Bar < Foo
            def add_1(x) # override
              x + 2
            end
          end

          obj = Bar.new
          p obj.add_1(1) #=> 3
          p Foo.instance_method(:add_1).bind(obj).call(1) #=> 2
          p Foo.instance_method(:add_1).bind_call(obj, 1) #=> 2

[Warning]

  [New methods]

    * Added Warning.[] and Warning.[]= to manage emitting/suppressing
      some categories of warnings.  [Feature #16345] [Feature #16420]

[$LOAD_PATH]

  [New method]

    * Added <code>$LOAD_PATH.resolve_feature_path</code>.  [Feature #15903] [Feature #15230]

=== Stdlib updates (outstanding ones only)

[Bundler]

  * Upgrade to Bundler 2.1.2.
    See https://github.com/bundler/bundler/releases/tag/v2.1.2

[CGI]

  * CGI.escapeHTML becomes 2~5x faster when there is at least one escaped character.
    See https://github.com/ruby/ruby/pull/2226

[CSV]

  * Upgrade to 3.1.2.
    See https://github.com/ruby/csv/blob/master/NEWS.md.

[Date]

  * Date.jisx0301, Date#jisx0301, and Date.parse support the new Japanese
    era.  [Feature #15742]

[Delegator]

  * Object#DelegateClass accepts a block and module_evals it in the context
    of the returned class, similar to Class.new and Struct.new.

[ERB]

  * Prohibit marshaling ERB instance.

[IRB]

  * Introduce syntax highlighting inspired by the Pry gem to Binding#irb
    source lines, REPL input, and inspect output of some core-class objects.

  * Introduce multiline editing mode provided by Reline.

  * Show documentation when completion.

  * Enable auto indent and save/load history by default.

[JSON]

  * Upgrade to 2.3.0.

[Net::FTP]

  * Add Net::FTP#features to check available features, and Net::FTP#option to
    enable/disable each of them.  [Feature #15964]

[Net::HTTP]

  * Add +ipaddr+ optional parameter to Net::HTTP#start to replace the address for
    the TCP/IP connection. [Feature #5180]

[Net::IMAP]

  * Add Server Name Indication (SNI) support.  [Feature #15594]

[open-uri]

  * Warn open-uri's "open" method at Kernel.
    Use URI.open instead.  [Misc #15893]

  * The default charset of "text/*" media type is UTF-8 instead of
    ISO-8859-1.  [Bug #15933]

[OptionParser]

  * Now show "Did you mean?" for unknown options.  [Feature #16256]

    test.rb:

      require "optparse"
      OptionParser.new do |opts|
        opts.on("-f", "--foo", "foo") {|v| }
        opts.on("-b", "--bar", "bar") {|v| }
        opts.on("-c", "--baz", "baz") {|v| }
      end.parse!

    example:

      $ ruby test.rb --baa
      Traceback (most recent call last):
      test.rb:7:in `<main>': invalid option: --baa (OptionParser::InvalidOption)
      Did you mean?  baz
                     bar

[Pathname]

  * Pathname.glob now delegates 3 arguments to Dir.glob
    to accept +base+ keyword. [Feature #14405]

[Racc]

  * Merge 1.4.15 from upstream repository and added cli of racc.

[Reline]

  * New stdlib that is compatible with the readline stdlib but is
    implemented in pure Ruby. It also provides a multiline editing mode.

[REXML]

  * Upgrade to 3.2.3.
    See https://github.com/ruby/rexml/blob/master/NEWS.md.

[RSS]

  * Upgrade to RSS 0.2.8.
    See https://github.com/ruby/rss/blob/master/NEWS.md.

[RubyGems]

  * Upgrade to RubyGems 3.1.2.
    * https://github.com/rubygems/rubygems/releases/tag/v3.1.0
    * https://github.com/rubygems/rubygems/releases/tag/v3.1.1
    * https://github.com/rubygems/rubygems/releases/tag/v3.1.2

[StringScanner]

  * Upgrade to 1.0.3.
    See https://github.com/ruby/strscan/blob/master/NEWS.md.

=== Compatibility issues (excluding feature bug fixes)

* The following libraries are no longer bundled gems.
  Install corresponding gems to use these features.
  * CMath (cmath gem)
  * Scanf (scanf gem)
  * Shell (shell gem)
  * Synchronizer (sync gem)
  * ThreadsWait (thwait gem)
  * E2MM (e2mmap gem)

[Proc]
  * The Proc#to_s format was changed. [Feature #16101]

[Range]
  * Range#minmax used to iterate on the range to determine the maximum.
    It now uses the same algorithm as Range#max. In rare cases (e.g.
    ranges of Floats or Strings), this may yield different results. [Bug #15807]

=== Stdlib compatibility issues (excluding feature bug fixes)

* Promote stdlib to default gems
  * The following default gems were published on rubygems.org
    * benchmark
    * cgi
    * delegate
    * getoptlong
    * net-pop
    * net-smtp
    * open3
    * pstore
    * readline
    * readline-ext
    * singleton
  * The following default gems were only promoted at ruby-core,
    but not yet published on rubygems.org.
    * monitor
    * observer
    * timeout
    * tracer
    * uri
    * yaml
* The <tt>did_you_mean</tt> gem has been promoted up to a default gem from a bundled gem

[pathname]

  * Kernel#Pathname when called with a Pathname argument now returns
    the argument instead of creating a new Pathname. This is more
    similar to other Kernel methods, but can break code that modifies
    the return value and expects the argument not to be modified.

[profile.rb, Profiler__]

  * Removed from standard library. It was unmaintained since Ruby 2.0.0.

=== C API updates

* Many <code>*_kw</code> functions have been added for setting whether
  the final argument being passed should be treated as keywords. You
  may need to switch to these functions to avoid keyword argument
  separation warnings, and to ensure correct behavior in Ruby 3.

* The <code>:</code> character in rb_scan_args format string is now
  treated as keyword arguments. Passing a positional hash instead of
  keyword arguments will emit a deprecation warning.

* C API declarations with +ANYARGS+ are changed not to use +ANYARGS+.
  See https://github.com/ruby/ruby/pull/2404

=== Implementation improvements

[Fiber]

  * Allow selecting different coroutine implementations by using
    +--with-coroutine=+, e.g.

         $ ./configure --with-coroutine=ucontext
         $ ./configure --with-coroutine=copy

  * Replace previous stack cache with fiber pool cache. The fiber pool
    allocates many stacks in a single memory region. Stack allocation
    becomes O(log N) and fiber creation is amortized O(1). Around 10x
    performance improvement was measured in micro-benchmarks.
    https://github.com/ruby/ruby/pull/2224

[File]
  * File.realpath now uses realpath(3) on many platforms, which can
    significantly improve performance. [Feature #15797]

[Hash]
  * Change data structure of small Hash objects. [Feature #15602]

[Monitor]
  * Monitor class is written in C-extension. [Feature #16255]

[Thread]

  * VM stack memory allocation is now combined with native thread stack,
    improving thread allocation performance and reducing allocation related
    failures. Around 10x performance improvement was measured in micro-benchmarks.

[JIT]

  * JIT-ed code is recompiled to less-optimized code when an optimization assumption is invalidated.

  * Method inlining is performed when a method is considered as pure.
    This optimization is still experimental and many methods are NOT considered as pure yet.

  * The default value of +--jit-max-cache+ is changed from 1,000 to 100.

  * The default value of +--jit-min-calls+ is changed from 5 to 10,000.

[RubyVM]

  * Per-call-site method cache, which has been there since around 1.9, was
    improved: cache hit rate raised from 89% to 94%.
    See https://github.com/ruby/ruby/pull/2583

[RubyVM::InstructionSequence]

  * RubyVM::InstructionSequence#to_binary method generates compiled binary.
    The binary size is reduced. [Feature #16163]

=== Miscellaneous changes

* Support for IA64 architecture has been removed. Hardware for testing was
  difficult to find, native fiber code is difficult to implement, and it added
  non-trivial complexity to the interpreter. [Feature #15894]

* Require compilers to support C99. [Misc #15347]

  * Details of our dialect: https://bugs.ruby-lang.org/projects/ruby-master/wiki/C99

* Ruby's upstream repository is changed from Subversion to Git.

  * https://git.ruby-lang.org/ruby.git

  * RUBY_REVISION class is changed from Integer to String.

  * RUBY_DESCRIPTION includes Git revision instead of Subversion's one.

* Support built-in methods in Ruby with the <code>_\_builtin_</code> syntax. [Feature #16254]

  Some methods are defined in *.rb (such as trace_point.rb).
  For example, it is easy to define a method which accepts keyword arguments.