We are pleased to announce the release of Ruby 2.7.0-rc2.

A release candidate is released to gather feedback for the final release
planned on December 25.

It also introduces a number of new features and performance improvements,
most notably:

* Pattern Matching
* REPL improvement
* Compaction GC
* Separation of positional and keyword arguments

## Pattern Matching [Experimental]

Pattern matching, a widely used feature in functional programming languages,
is introduced as an experimental feature.
[[Feature #14912]](https://bugs.ruby-lang.org/issues/14912)

It can traverse a given object and assign its value if it matches a pattern.

```ruby
require "json"

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

case JSON.parse(json, symbolize_names: true)
in {name: "Alice", children: [{name: "Bob", age: age}]}
  p age #=> 2
end
```

For more details, please see
[Pattern matching - New feature in Ruby
2.7](https://speakerdeck.com/k_tsj/pattern-matching-new-feature-in-ruby-2-dot-7).

## REPL improvement

`irb`, the bundled interactive environment (REPL; Read-Eval-Print-Loop),
now supports multi-line editing. It is powered by `reline`,
a `readline`-compatible pure Ruby implementation.
It also provides rdoc integration. In `irb` you can display the reference
for a given class, module, or method.
[[Feature #14683]](https://bugs.ruby-lang.org/issues/14683),
[[Feature #14787]](https://bugs.ruby-lang.org/issues/14787),
[[Feature #14918]](https://bugs.ruby-lang.org/issues/14918)

Besides, source lines shown at `binding.irb` and inspect results
for core-class objects are now colorized.

<video autoplay="autoplay" controls="controls" muted="muted"
width="576" height="259">
  <source src="https://cache.ruby-lang.org/pub/media/irb_improved_with_key_take2.mp4"
type="video/mp4">
</video>

## Compaction GC

This release introduces Compaction GC which can defragment
a fragmented memory space.

Some multi-threaded Ruby programs may cause memory fragmentation,
leading to high memory usage and degraded speed.

The `GC.compact` method is introduced 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 friendly.
[[Feature #15626]](https://bugs.ruby-lang.org/issues/15626)

## Separation of positional and keyword arguments

Automatic conversion of keyword arguments and positional arguments
is deprecated, and conversion will be removed in Ruby 3.
[[Feature #14183]](https://bugs.ruby-lang.org/issues/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.

  ```ruby
  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.

  ```ruby
  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.

  ```ruby
  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.

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

* Non-symbols are allowed as keyword argument keys if the method accepts
  arbitrary keywords.
  [[Feature #14183]](https://bugs.ruby-lang.org/issues/14183)

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

* `**nil` 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]](https://bugs.ruby-lang.org/issues/14183)

  ```ruby
  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]](https://bugs.ruby-lang.org/issues/14183)

  ```ruby
  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)    # {}
  ```

NOTE: Too many deprecation warnings about keyword argument incompatibility
have been pointed out to be too verbose.  Currently, two possible solutions
are discussed; disabling deprecation warnings by default
([#16345](https://bugs.ruby-lang.org/issues/16345))
or suppressing duplicated warnings
([#16289](https://bugs.ruby-lang.org/issues/16289)).
The final decision is not made, but will be fixed by the official release.

## Other Notable New Features

* A method reference operator, `.:`, was introduced
  as an experimental feature in earlier previews, but was reverted.
  [[Feature #12125]](https://bugs.ruby-lang.org/issues/12125),
  [[Feature #13581]]( https://bugs.ruby-lang.org/issues/13581),
  [[Feature #16275]](https://bugs.ruby-lang.org/issues/16275)

* Numbered parameter as the default block parameter is introduced
  as an experimental feature.
  [[Feature #4475]](https://bugs.ruby-lang.org/issues/4475)

* A beginless range is experimentally introduced.  It might not be
  as useful as an endless range, but would be good for DSL purposes.
  [[Feature #14799]](https://bugs.ruby-lang.org/issues/14799)

  ```ruby
  ary[..3]  # identical to ary[0..3]
  rel.where(sales: ..100)
  ```

* `Enumerable#tally` is added.  It counts the occurrence of each element.

  ```ruby
  ["a", "b", "c", "b"].tally
  #=> {"a"=>1, "b"=>2, "c"=>1}
  ```

* Calling a private method on `self` is now allowed.
  [[Feature #11297]](https://bugs.ruby-lang.org/issues/11297),
  [[Feature #16123]](https://bugs.ruby-lang.org/issues/16123)

  ```ruby
  def foo
  end
  private :foo
  self.foo
  ```

* `Enumerator::Lazy#eager` is added.
  It generates a non-lazy enumerator from a lazy enumerator.
  [[Feature #15901]](https://bugs.ruby-lang.org/issues/15901)

  ```ruby
  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!?"]
  ```

## Performance improvements

* JIT [Experimental]

  * 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-min-calls` is changed from 5 to 10,000.

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

* `Module#name`, `true.to_s`, `false.to_s`,
  and `nil.to_s` now always return a frozen String.
  The returned String is always the same for a given object.
  [Experimental]
  [[Feature #16150]](https://bugs.ruby-lang.org/issues/16150)

* The performance of `CGI.escapeHTML` is improved.
  [GH-2226](https://github.com/ruby/ruby/pull/2226)

* The performance of Monitor and MonitorMixin is improved.
  [[Feature #16255]](https://bugs.ruby-lang.org/issues/16255)

## Other notable changes since 2.6

* Some standard libraries are updated.
  * Bundler 2.1.0.pre.3
    ([History](https://github.com/bundler/bundler/blob/2-1-stable/CHANGELOG.md#210pre3-november-8-2019))
  * RubyGems 3.1.0.pre.3
    ([History](https://github.com/rubygems/rubygems/blob/3.1/History.txt))
  * CSV 3.1.2
    ([NEWS](https://github.com/ruby/csv/blob/v3.1.2/NEWS.md))
  * Racc 1.4.15
  * REXML 3.2.3
    ([NEWS](https://github.com/ruby/rexml/blob/v3.2.3/NEWS.md))
  * RSS 0.2.8
    ([NEWS](https://github.com/ruby/rss/blob/v0.2.8/NEWS.md))
  * StringScanner 1.0.3
  * Some other libraries that have no original version are also updated.

* Promote stdlib to default gems
  * The following default gems were published on rubygems.org
    * benchmark
    * cgi
    * delegate
    * getoptlong
    * net-pop
    * net-smtp
    * open3
    * pstore
    * singleton
  * The following default gems were only promoted at ruby-core,
    but not yet published on rubygems.org.
    * monitor
    * observer
    * timeout
    * tracer
    * uri
    * yaml

* `Proc.new` and `proc` with no block in a method called with a block
   is warned now.

* `lambda` with no block in a method called with a block errs.

* Update Unicode version and Emoji version from 11.0.0 to 12.0.0.
  [[Feature #15321]](https://bugs.ruby-lang.org/issues/15321)

* Update Unicode version to 12.1.0,
  adding support for U+32FF SQUARE ERA NAME REIWA.
  [[Feature #15195]](https://bugs.ruby-lang.org/issues/15195)

* `Date.jisx0301`, `Date#jisx0301`, and `Date.parse` support the
  new Japanese era.
  [[Feature #15742]](https://bugs.ruby-lang.org/issues/15742)

* Require compilers to support C99
  [[Misc #15347]](https://bugs.ruby-lang.org/issues/15347)
  * Details of our dialect:
    <https://bugs.ruby-lang.org/projects/ruby-trunk/wiki/C99>

See [NEWS](https://github.com/ruby/ruby/blob/v2_7_0_rc2/NEWS)
or [commit logs](https://github.com/ruby/ruby/compare/v2_6_0...v2_7_0_rc2)
for more details.

With those changes, [4184 files changed, 226864 insertions(+), 99937
deletions(-)](https://github.com/ruby/ruby/compare/v2_6_0...v2_7_0_rc2)
since Ruby 2.6.0!
Enjoy programming with Ruby 2.7!

## Download

* <https://cache.ruby-lang.org/pub/ruby/2.7/ruby-2.7.0-rc2.tar.bz2>

      SIZE: 14686646
      SHA1: e04680f57d8b7576637eb75b8b56aceeb1806992
      SHA256: 8f94ea7ba79b6e95225fb4a7870e882081182c3d12d58c4cad2a7d2e7865cf8e
      SHA512: 9010f72bb3f33b6cd3f515531e6e05198f295bb2a8a788e3a46cdfd776a9f6176b6ba8612f07f0236a11359302d2b77fdecca1dc6be33581edbb028069397a0a

* <https://cache.ruby-lang.org/pub/ruby/2.7/ruby-2.7.0-rc2.tar.gz>

      SIZE: 16775053
      SHA1: 787a86023f0abe6ca9c0b31e95328725e8bb7814
      SHA256: b16cd92479e5648cc53425602e9dc6d76b18dd2cc180add2fd4c9f254646779d
      SHA512: d59910a140ea1b7ca7a64073dbbe4cbe8f11cd6fc68ea7874ca160e1a23549bd159f49f4d199002f9806e77d4426bff3aa81b62707d539e0710ece7b7ff83438

* <https://cache.ruby-lang.org/pub/ruby/2.7/ruby-2.7.0-rc2.tar.xz>

      SIZE: 11965624
      SHA1: 1f9f30eaf1829250931c4c465ee1c15e07452e7d
      SHA256: c90d29fba655b2dd577ff755f084e4d1fe0673cfcd888af7ff5d0b2d2f449bb7
      SHA512: dba23aada4921c98eb90d216db656833d1759c4f611d5087e2a0123d932ab1c6704dfedc0d671d2d51b4b3949ff95b6aec012481141c6fce3988a3d0bc5d18b8

* <https://cache.ruby-lang.org/pub/ruby/2.7/ruby-2.7.0-rc2.zip>

      SIZE: 20642713
      SHA1: e0b6f91398d55436b776d7a5eae0faaf810b1578
      SHA256: ac87c1666cc840cad26083a067bae1975d1fdb41ca1f1569903c05bca1b61174
      SHA512: 4e84b1f59b574a59b5346d30a0770e06ad81a4838813cc8789157f4e1a3fcbe7ca75bf83663c20736024760f1b0675ca288f1cee7f8a28f8918c4e43b0d09982

## What is Ruby

Ruby was first developed by Matz (Yukihiro Matsumoto) in 1993,
and is now developed as Open Source. It runs on multiple platforms
and is used all over the world especially for web development.

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>