Ruby 4.0 will be released soon. This feature checklist is based on the NEWS file from CRuby's master branch. We'll update this list until the release has been finalized.

Pull requests to implement Ruby 4.0 features should be based on the ruby-4.0 branch, not master.

NEWS for Ruby 4.0

This document is a list of user-visible feature changes
since the 3.4.0 release, except for bug fixes.

Note that each entry is kept to a minimum, see links for details.

Language changes

• *nil no longer calls nil.to_a, similar to how **nil does
  not call nil.to_hash. [Feature #21047] (Avoid array allocation for *nil, by not calling nil.to_a #9074)

• Logical binary operators (||, &&, and and or) at the
  beginning of a line continue the previous line, like fluent dot.
  The following two code are equal:

  if condition1
     && condition2
    ...
  end

  if condition1 && condition2
    ...
  end

  [Feature #20925]

Core classes updates

Note: We're only listing outstanding class updates.

• Array

  • Array#rfind has been added as a more efficient alternative to array.reverse_each.find [Feature #21678]
  • Array#find has been added as a more efficient override of Enumerable#find [Feature #21678]

• Binding

  • Binding#local_variables does no longer include numbered parameters.
    Also, Binding#local_variable_get, Binding#local_variable_set, and
    Binding#local_variable_defined? reject to handle numbered parameters.
    [Bug #21049]

  • Binding#implicit_parameters, Binding#implicit_parameter_get, and
    Binding#implicit_parameter_defined? have been added to access
    numbered parameters and "it" parameter. [Bug #21049]

• Enumerator

  • Enumerator.produce now accepts an optional size keyword argument
    to specify the size of the enumerator. It can be an integer,
    Float::INFINITY, a callable object (such as a lambda), or nil to
    indicate unknown size. When not specified, the size defaults to
    Float::INFINITY.

    # Infinite enumerator
    enum = Enumerator.produce(1, size: Float::INFINITY, &:succ)
    enum.size  # => Float::INFINITY
    
    # Finite enumerator with known/computable size
    abs_dir = File.expand_path("./baz") # => "/foo/bar/baz"
    traverser = Enumerator.produce(abs_dir, size: -> { abs_dir.count("/") + 1 }) {
      raise StopIteration if it == "/"
      File.dirname(it)
    }
    traverser.size  # => 4

    [Feature #21701]

• Fiber

  • Introduce support for Fiber#raise(cause:) argument similar to
    Kernel#raise. [Feature #21360]

• Fiber::Scheduler

  • Introduce Fiber::Scheduler#fiber_interrupt to interrupt a fiber with a
    given exception. The initial use case is to interrupt a fiber that is
    waiting on a blocking IO operation when the IO operation is closed.
    [Feature #21166]

  • Introduce Fiber::Scheduler#yield to allow the fiber scheduler to
    continue processing when signal exceptions are disabled.
    [Bug #21633]

  • Reintroduce the Fiber::Scheduler#io_close hook for asynchronous IO#close.

  • Invoke Fiber::Scheduler#io_write when flushing the IO write buffer.
    [Bug #21789]

• File

  • File::Stat#birthtime is now available on Linux via the statx
    system call when supported by the kernel and filesystem.
    [Feature #21205]

• IO

  • IO.select accepts Float::INFINITY as a timeout argument.
    [Feature #20610] (Align IO.select timeout logic with CRuby #9079)

  • A deprecated behavior, process creation by IO class methods
    with a leading |, was removed. [Feature #19630]

• Kernel

  • Kernel#inspect now checks for the existence of a #instance_variables_to_inspect method,
    allowing control over which instance variables are displayed in the #inspect string:

    class DatabaseConfig
      def initialize(host, user, password)
        @host = host
        @user = user
        @password = password
      end
    
      private def instance_variables_to_inspect = [:@host, :@user]
    end
    
    conf = DatabaseConfig.new("localhost", "root", "hunter2")
    conf.inspect #=> #<DatabaseConfig:0x0000000104def350 @host="localhost", @user="root">

    [Feature #21219] (Selective inspect of instance variables #9075)

  • A deprecated behavior, process creation by Kernel#open with a
    leading |, was removed. [Feature #19630]

• Math

  • Math.log1p and Math.expm1 are added. [Feature #21527] (Add Math.{expm1,log1p} #9078)

• Pathname

  • Pathname has been promoted from a default gem to a core class of Ruby.
    [Feature #17473]

• Proc

  • Proc#parameters now shows anonymous optional parameters as [:opt]
    instead of [:opt, nil], making the output consistent with when the
    anonymous parameter is required. [Bug #20974]

• Ractor

  • Ractor::Port class was added for a new synchronization mechanism
    to communicate between Ractors. [Feature #21262]

    port1 = Ractor::Port.new
    port2 = Ractor::Port.new
    Ractor.new port1, port2 do |port1, port2|
      port1 << 1
      port2 << 11
      port1 << 2
      port2 << 12
    end
    2.times{ p port1.receive } #=> 1, 2
    2.times{ p port2.receive } #=> 11, 12

    Ractor::Port provides the following methods:

    • Ractor::Port#receive
    • Ractor::Port#send (or Ractor::Port#<<)
    • Ractor::Port#close
    • Ractor::Port#closed?

    As result, Ractor.yield and Ractor#take were removed.

  • Ractor#join and Ractor#value were added to wait for the
    termination of a Ractor. These are similar to Thread#join
    and Thread#value.

  • Ractor#monitor and Ractor#unmonitor were added as low-level
    interfaces used internally to implement Ractor#join.

  • Ractor.select now only accepts Ractors and Ports. If Ractors are given,
    it returns when a Ractor terminates.

  • Ractor#default_port was added. Each Ractor has a default port,
    which is used by Ractor.send, Ractor.receive.

  • Ractor#close_incoming and Ractor#close_outgoing were removed.

  • Ractor.shareable_proc and Ractor.shareable_lambda is introduced
    to make shareable Proc or lambda.
    [Feature #21550], [Feature #21557]

• Range

  • Range#to_set now performs size checks to prevent issues with
    endless ranges. [Bug #21654]

  • Range#overlap? now correctly handles infinite (unbounded) ranges.
    [Bug #21185]

  • Range#max behavior on beginless integer ranges has been fixed.
    [Bug #21174] [Bug #21175]

• Ruby

  • A new toplevel module Ruby has been defined, which contains
    Ruby-related constants. This module was reserved in Ruby 3.4
    and is now officially defined. [Feature #20884]

• Ruby::Box

  • A new (experimental) feature to provide separation about definitions.
    For the detail of "Ruby Box", see doc/language/box.md.
    [Feature #21311] [Misc #21385]

• Set

  • Set is now a core class, instead of an autoloaded stdlib class.
    [Feature #21216] (Move Set to a core class [Ruby 4.0] #9066)

  • Set#inspect now uses a simpler display, similar to literal arrays.
    (e.g., Set[1, 2, 3] instead of #<Set: {1, 2, 3}>). [Feature #21389]

  • Passing arguments to Set#to_set and Enumerable#to_set is now deprecated.
    [Feature #21390]

• Socket

  • Socket.tcp & TCPSocket.new accepts open_timeout as a keyword argument to specify
    the timeout for the initial connection. [Feature #21347]
  • When a user-specified timeout occurred in TCPSocket.new, either Errno::ETIMEDOUT
    or IO::TimeoutError could previously be raised depending on the situation.
    This behavior has been unified so that IO::TimeoutError is now consistently raised.
    (Please note that, in Socket.tcp, there are still cases where Errno::ETIMEDOUT
    may be raised in similar situations, and that in both cases Errno::ETIMEDOUT may be
    raised when the timeout occurs at the OS level.)

• String

  • Update Unicode to Version 17.0.0 and Emoji Version 17.0 #9250
    [Feature #19908][Feature #20724] (also applies to Regexp)

  • String#strip, strip!, lstrip, lstrip!, rstrip, and rstrip!
    are extended to accept *selectors arguments. [Feature #21552] (String#split and rsplit do not support selectors (Ruby 4.0) #9267)

• Thread

  • Introduce support for Thread#raise(cause:) argument similar to
    Kernel#raise. [Feature #21360]

Stdlib updates

The following bundled gems are promoted from default gems.

• ostruct 0.6.3
• pstore 0.2.0
• benchmark 0.5.0
• logger 1.7.0
• rdoc 7.0.3
• win32ole 1.9.2
• irb 1.16.0
• reline 0.6.3
• readline 0.0.4
• fiddle 1.1.8

We only list stdlib changes that are notable feature changes.

Other changes are listed in the following sections. We also listed release
history from the previous bundled version that is Ruby 3.3.0 if it has GitHub
releases.

The following default gem is added.

• win32-registry 0.1.1

The following default gems are updated.

• RubyGems 4.0.3
• bundler 4.0.3
• date 3.5.1
• delegate 0.6.1
• digest 3.2.1
• english 0.8.1
• erb 6.0.1
• error-highlight 0.7.1
• etc 1.4.6
• fcntl 1.3.0
• fileutils 1.8.0
• forwardable 1.4.0
• io-console 0.8.2
• io-nonblock 0.3.2
• io-wait 0.4.0
• ipaddr 1.2.8
• json 2.18.0
• net-http 0.9.1
• openssl 4.0.0
• optparse 0.8.1
• pp 0.6.3
• prism 1.6.0
• psych 5.3.1
• resolv 0.7.0
• stringio 3.2.0
• strscan 3.1.6
• time 0.4.2
• timeout 0.4.4
• uri 1.1.1
• weakref 0.1.4
• zlib 3.2.2

The following bundled gems are added.

The following bundled gems are updated.

• minitest 6.0.0
• power_assert 3.0.1
• rake 13.3.1
• test-unit 3.7.5
• rexml 3.4.4
• rss 0.3.2
• net-ftp 0.3.9
• net-imap 0.6.2
• net-smtp 0.5.1
• matrix 0.4.3
• prime 0.1.4
• rbs 3.10.0
• typeprof 0.31.1
• debug 1.11.1
• base64 0.3.0
• bigdecimal 4.0.1
• drb 2.2.3
• syslog 0.3.0
• csv 3.3.5
• repl_type_completor 0.1.12

Supported platforms

• Windows

  • Dropped support for MSVC versions older than 14.0 (_MSC_VER 1900).
    This means Visual Studio 2015 or later is now required.

Compatibility issues

• The following methods were removed from Ractor due because of Ractor::Port:

  • Ractor.yield
  • Ractor#take
  • Ractor#close_incoming
  • Ractor#close_outgoging

  [Feature #21262]

• ObjectSpace._id2ref is deprecated. [Feature #15408]

• Process::Status#& and Process::Status#>> have been removed.
  They were deprecated in Ruby 3.3. [Bug #19868]

• rb_path_check has been removed. This function was used for
  $SAFE path checking which was removed in Ruby 2.7,
  and was already deprecated.
  [Feature #20971]

• A backtrace for ArgumentError of "wrong number of arguments" now
  include the receiver's class or module name (e.g., in Foo#bar
  instead of in bar). [Bug #21698]

• Backtraces no longer display internal frames.
  These methods now appear as if it is in the Ruby source file,
  consistent with other C-implemented methods. [Bug #20968]

  Before:

  ruby -e '[1].fetch_values(42)'
  <internal:array>:211:in 'Array#fetch': index 42 outside of array bounds: -1...1 (IndexError)
          from <internal:array>:211:in 'block in Array#fetch_values'
          from <internal:array>:211:in 'Array#map!'
          from <internal:array>:211:in 'Array#fetch_values'
          from -e:1:in '<main>'
  

  After:

  $ ruby -e '[1].fetch_values(42)'
  -e:1:in 'Array#fetch_values': index 42 outside of array bounds: -1...1 (IndexError)
          from -e:1:in '<main>'
  

Stdlib compatibility issues

• CGI library is removed from the default gems. Now we only provide cgi/escape for
  the following methods:

  • CGI.escape and CGI.unescape
  • CGI.escapeHTML and CGI.unescapeHTML
  • CGI.escapeURIComponent and CGI.unescapeURIComponent
  • CGI.escapeElement and CGI.unescapeElement

  [Feature #21258]

• With the move of Set from stdlib to core class, set/sorted_set.rb has
  been removed, and SortedSet is no longer an autoloaded constant. Please
  install the sorted_set gem and require 'sorted_set' to use SortedSet.
  [Feature #21287]

• Net::HTTP

  • The default behavior of automatically setting the Content-Type header
    to application/x-www-form-urlencoded for requests with a body
    (e.g., POST, PUT) when the header was not explicitly set has been
    removed. If your application relied on this automatic default, your
    requests will now be sent without a Content-Type header, potentially
    breaking compatibility with certain servers.
    [GH-net-http #205]

Implementation improvements

Ractor

A lot of work has gone into making Ractors more stable, performant, and usable. These improvements bring Ractors implementation closer to leaving experimental status.

• Performance improvements
  • Frozen strings and the symbol table internally use a lock-free hash set
  • Method cache lookups avoid locking in most cases
  • Class (and geniv) instance variable access is faster and avoids locking
  • Cache contention is avoided during object allocation
  • object_id avoids locking in most cases
• Bug fixes and stability
  • Fixed possible deadlocks when combining Ractors and Threads
  • Fixed issues with require and autoload in a Ractor
  • Fixed encoding/transcoding issues across Ractors
  • Fixed race conditions in GC operations and method invalidation
  • Fixed issues with processes forking after starting a Ractor
  • GC allocation counts are now accurate under Ractors
  • Fixed TracePoints not working after GC [Bug #19112]