# NEWS for Ruby 3.1.0

This document is a list of user-visible feature changes
since the **3.0.0** release, except for bug fixes.

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

## Language changes

*   The block argument can now be anonymous if the block will
    only be passed to another method. [[Feature #11256]]

    ```ruby
    def foo(&)
      bar(&)
    end
    ```

*   Pin operator now takes an expression. [[Feature #17411]]

    ```ruby
    Prime.each_cons(2).lazy.find_all{_1 in [n, ^(n + 2)]}.take(3).to_a
    #=> [[3, 5], [5, 7], [11, 13]]
    ```

*   Pin operator now supports instance, class, and global variables.
    [[Feature #17724]]

    ```ruby
    @n = 5
    Prime.each_cons(2).lazy.find{_1 in [n, ^@n]}
    #=> [3, 5]
    ```

*   One-line pattern matching is no longer experimental.

*   Parentheses can be omitted in one-line pattern matching.
    [[Feature #16182]]

    ```ruby
    [0, 1] => _, x
    {y: 2} => y:
    x #=> 1
    y #=> 2
    ```

*   Multiple assignment evaluation order has been made consistent with
    single assignment evaluation order.  With single assignment, Ruby
    uses a left-to-right evaluation order.  With this code:

    ```ruby
    foo[0] = bar
    ```

    The following evaluation order is used:

    1. `foo`
    2. `bar`
    3. `[]=` called on the result of `foo`

    In Ruby before 3.1.0, multiple assignment did not follow this
    evaluation order.  With this code:

    ```ruby
    foo[0], bar.baz = a, b
    ```

    Versions of Ruby before 3.1.0 would evaluate in the following
    order

    1. `a`
    2. `b`
    3. `foo`
    4. `[]=` called on the result of `foo`
    5. `bar`
    6. `baz=` called on the result of `bar`

    Starting in Ruby 3.1.0, the evaluation order is now consistent with
    single assignment, with the left-hand side being evaluated before
    the right-hand side:

    1. `foo`
    2. `bar`
    3. `a`
    4. `b`
    5. `[]=` called on the result of `foo`
    6. `baz=` called on the result of `bar`

    [[Bug #4443]]

*   Values in Hash literals and keyword arguments can be omitted.
    [[Feature #14579]]

    For example,

    * `{x:, y:}` is a syntax sugar of `{x: x, y: y}`.
    * `foo(x:, y:)` is a syntax sugar of `foo(x: x, y: y)`.

    Constant names, local variable names, and method names are allowed as
    key names.  Note that a reserved word is considered as a local
    variable or method name even if it's a pseudo variable name such as
    `self`.

*   Non main-Ractors can get instance variables (ivars) of classes/modules
    if ivars refer to shareable objects.
    [[Feature #17592]]

*   A command syntax is allowed in endless method definitions, i.e.,
    you can now write `def foo = puts "Hello"`.
    Note that `private def foo = puts "Hello"` does not parse.
    [[Feature #17398]]

## Command line options

* `--disable-gems` is now explicitly declared as "just for debugging".
  Never use it in any real-world codebase.
  [[Feature #17684]]

## Core classes updates

Note: We're only listing outstanding class updates.

* Array

    * Array#intersect? is added. [[Feature #15198]]

* Class

    *   Class#descendants, which returns an array of classes
        directly or indirectly inheriting from the receiver, not
        including the receiver or singleton classes.
        [[Feature #14394]]

        ```ruby
        class A; end
        class B < A; end
        class C < B; end
        A.descendants    #=> [B, C]
        B.descendants    #=> [C]
        C.descendants    #=> []
        ```

    *   Class#subclasses, which returns an array of classes
        directly inheriting from the receiver, not
        including singleton classes.
        [[Feature #18273]]

        ```ruby
        class A; end
        class B < A; end
        class C < B; end
        class D < A; end
        A.subclasses    #=> [D, B]
        B.subclasses    #=> [C]
        C.subclasses    #=> []
        ```

* Enumerable

    *   Enumerable#compact is added. [[Feature #17312]]

    *   Enumerable#tally now accepts an optional hash to count. [[Feature #17744]]

    *   Enumerable#each_cons and each_slice to return a receiver. [[GH-1509]]

        ```ruby
        [1, 2, 3].each_cons(2){}
        # 3.0 => nil
        # 3.1 => [1, 2, 3]

        [1, 2, 3].each_slice(2){}
        # 3.0 => nil
        # 3.1 => [1, 2, 3]
        ```

* Enumerator::Lazy

    *   Enumerator::Lazy#compact is added. [[Feature #17312]]

* File

    *   File.dirname now accepts an optional argument for the level to
        strip path components. [[Feature #12194]]

* GC

    *   "GC.measure_total_time = true" enables the measurement of GC.
        Measurement can introduce overhead. It is enabled by default.
        GC.measure_total_time returns the current setting.
        GC.stat[:time] or GC.stat(:time) returns measured time
        in milli-seconds.

    *   GC.total_time returns measured time in nano-seconds.

* Integer

    *   Integer.try_convert is added. [[Feature #15211]]

* Kernel


    *   Kernel#load now accepts a module as the second argument,
        and will load the file using the given module as the
        top-level module. [[Feature #6210]]

* Marshal

    *   Marshal.load now accepts a `freeze: true` option.
        All returned objects are frozen except for `Class` and
        `Module` instances. Strings are deduplicated. [[Feature #18148]]

* MatchData

    *   MatchData#match is added [[Feature #18172]]

    *   MatchData#match_length is added [[Feature #18172]]

* Method/UnboundMethod

    *   Method#public?, Method#private?, Method#protected?,
        UnboundMethod#public?, UnboundMethod#private?,
        UnboundMethod#protected? have been added. [[Feature #11689]]

* Module

    *   Module#prepend now modifies the ancestor chain if the receiver
        already includes the argument. Module#prepend still does not
        modify the ancestor chain if the receiver has already prepended
        the argument. [[Bug #17423]]

    *   Module#private, #public, #protected, and #module_function will
        now return their arguments.  If a single argument is given, it
        is returned. If no arguments are given, nil is returned.  If
        multiple arguments are given, they are returned as an array.
        [[Feature #12495]]

* Process

    *   Process.\_fork is added. This is a core method for fork(2).
        Do not call this method directly; it is called by existing
        fork methods: Kernel.#fork, Process.fork, and IO.popen("-").
        Application monitoring libraries can overwrite this method to
        hook fork events. [[Feature #17795]]

* Struct

    *   Passing only keyword arguments to Struct#initialize is warned.
        You need to use a Hash literal to set a Hash to a first member.
        [[Feature #16806]]

    *   StructClass#keyword_init? is added [[Feature #18008]]

* String

    *   Update Unicode version to 13.0.0 [[Feature #17750]]
        and Emoji version to 13.0 [[Feature #18029]]

    *   String#unpack and String#unpack1 now accept an `offset:` keyword
        argument to start the unpacking after an arbitrary number of bytes
        have been skipped. If `offset` is outside of the string bounds
        `ArgumentError` is raised. [[Feature #18254]]

* Queue

    *   Queue#initialize now accepts an Enumerable of initial values.
        [[Feature #17327]]

* Thread

    *   Thread#native_thread_id is added. [[Feature #17853]]

* Thread::Backtrace

    *   Thread::Backtrace.limit, which returns the value to limit backtrace
        length set by `--backtrace-limit` command line option, is added.
        [[Feature #17479]]

* Time

    *   Time.new now accepts optional `in:` keyword argument for the
        timezone, as well as `Time.at` and `Time.now`, so that is now
        you can omit minor arguments to `Time.new`. [[Feature #17485]]

    *   Time#strftime supports RFC 3339 UTC for unknown offset local
        time, `-0000`, as `%-z`. [[Feature #17544]]

* TracePoint

    *   TracePoint.allow_reenter is added to allow reenter while TracePoint
        callback.
        [[Feature #15912]]

* $LOAD_PATH

    *   $LOAD_PATH.resolve_feature_path does not raise. [[Feature #16043]]

* Fiber Scheduler

    *   Add support for `Addrinfo.getaddrinfo` using `address_resolve` hook.
        [[Feature #17370]]

    *   Introduce non-blocking `Timeout.timeout` using `timeout_after` hook.
        [[Feature #17470]]

    *   Introduce new scheduler hooks `io_read` and `io_write` along with a
        low level `IO::Buffer` for zero-copy read/write. [[Feature #18020]]

    *   IO hooks `io_wait`, `io_read`, `io_write`, receive the original IO object
        where possible. [[Bug #18003]]

    *   Make `Monitor` fiber-safe. [[Bug #17827]]

    *   Replace copy coroutine with pthread implementation. [[Feature #18015]]

* Refinement

    *   New class which represents a module created by Module#refine.
        `include` and `prepend` are deprecated, and `import_methods` is added
        instead. [[Bug #17429]]

## Stdlib updates

*   The following default gem are updated.
    * RubyGems 3.3.0.dev
    * base64 0.1.1
    * benchmark 0.2.0
    * bigdecimal 3.1.0.dev
    * bundler 2.3.0.dev
    * cgi 0.3.1
    * csv 3.2.1
    * date 3.2.2
    * did_you_mean 1.6.0.pre.alpha
    * digest 3.1.0.pre3
    * drb 2.1.0
    * erb 2.2.3
    * error_highlight 0.2.0
    * etc 1.3.0
    * fcntl 1.0.1
    * fiddle 1.1.0
    * fileutils 1.6.0
    * find 0.1.1
    * io-console 0.5.9
    * io-wait 0.2.1
    * ipaddr 1.2.3
    * irb 1.3.8.pre.11
    * json 2.6.1
    * logger 1.4.4
    * net-http 0.2.0
    * net-protocol 0.1.2
    * nkf 0.1.1
    * open-uri 0.2.0
    * openssl 3.0.0.pre
    * optparse 0.2.0
    * ostruct 0.5.2
    * pathname 0.2.0
    * pp 0.3.0
    * prettyprint 0.1.1
    * psych 4.0.2
    * racc 1.6.0
    * rdoc 6.3.2
    * readline-ext 0.1.3
    * reline 0.2.8.pre.11
    * resolv 0.2.1
    * rinda 0.1.1
    * ruby2_keywords 0.0.5
    * securerandom 0.1.1
    * set 1.0.2
    * stringio 3.0.1
    * strscan 3.0.1
    * tempfile 0.1.2
    * time 0.2.0
    * timeout 0.2.0
    * tmpdir 0.1.2
    * un 0.2.0
    * uri 0.11.0
    * yaml 0.2.0
    * zlib 2.1.1
*   The following bundled gems are updated.
    * minitest 5.15.0
    * power_assert 2.0.1
    * rake 13.0.6
    * test-unit 3.5.3
    * rexml 3.2.5
    * rbs 1.8.1
    * typeprof 0.20.4
*   The following default gems are now bundled gems.
    * net-ftp 0.1.3
    * net-imap 0.2.2
    * net-pop 0.1.1
    * net-smtp 0.3.1
    * matrix 0.4.2
    * prime 0.1.2
    * debug 1.4.0

* Coverage measurement now supports suspension. You can use `Coverage.suspend`
  to stop the measurement temporarily, and `Coverage.resume` to restart it.
  See [[Feature #18176]] in detail.

* Random::Formatter is moved to random/formatter.rb, so that you can
  use `Random#hex`, `Random#base64`, and so on without SecureRandom.
  [[Feature #18190]]

## Compatibility issues

Note: Excluding feature bug fixes.

* `rb_io_wait_readable`, `rb_io_wait_writable` and `rb_wait_for_single_fd` are
  deprecated in favour of `rb_io_maybe_wait_readable`,
  `rb_io_maybe_wait_writable` and `rb_io_maybe_wait` respectively.
  `rb_thread_wait_fd` and `rb_thread_fd_writable` are deprecated. [[Bug #18003]]

## Stdlib compatibility issues

* `ERB#initialize` warns `safe_level` and later arguments even without -w.
  [[Feature #14256]]

* `lib/debug.rb` is replaced with `debug.gem`

* `Kernel#pp` in `lib/pp.rb` uses the width of `IO#winsize` by default.
  This means that the output width is automatically changed depending on
  your terminal size. [[Feature #12913]]

## C API updates

* Documented. [[GH-4815]]

* `rb_gc_force_recycle` is deprecated and has been changed to a no-op.
  [[Feature #18290]]

## Implementation improvements

* `instance_eval` and `instance_exec` now only allocate a singleton class when
  required, avoiding extra objects and improving performance. [[GH-5146]]

* The performance of `Struct` accessors is improved. [[GH-5131]]

* `mandatory_only?` builtin special form to improve performance on
  builtin methods. [[GH-5112]]

* Experimental feature Variable Width Allocation in the garbage collector.
  This feature is turned off by default and can be enabled by compiling Ruby
  with flag `USE_RVARGC=1` set. [[Feature #18045]] [[Feature #18239]]

## JIT

* Rename Ruby 3.0's `--jit` to `--mjit`, and alias `--jit` to `--yjit`
  on non-Windows x86-64 platforms and to `--mjit` on others.

### MJIT

* The default `--mjit-max-cache` is changed from 100 to 10000.

* JIT-ed code is no longer cancelled when a TracePoint for class events
  is enabled.

* The JIT compiler no longer skips compilation of methods longer than
  1000 instructions.

* `--mjit-verbose` and `--mjit-warning` output "JIT cancel" when JIT-ed
  code is disabled because TracePoint or GC.compact is used.

### YJIT: New experimental in-process JIT compiler

New JIT compiler available as an experimental feature. [[Feature #18229]]

See [this blog post](https://shopify.engineering/yjit-just-in-time-compiler-cruby
) introducing the project.

* Disabled by default, use `--yjit` command-line option to enable YJIT.

* Performance improvements on most real-world software, up to 22% on railsbench, 39% on liquid-render.

* Fast warm-up times.

* Limited to macOS & Linux on x86-64 platforms for now.

## Static analysis

### RBS

* [rbs collection](https://github.com/ruby/rbs/blob/master/docs/collection.md) has been introduced to manage gems’ RBSs.
* Many signatures for built-in and standard libraries have been added/updated.
* It includes many bug fixes and performance improvements too.

See the [CHANGELOG.md](https://github.com/ruby/rbs/blob/master/CHANGELOG.md) for more information.

### TypeProf

* [Experimental IDE support](https://github.com/ruby/typeprof/blob/master/doc/ide.md) has been implemented.
* Many bug fixes and performance improvements since Ruby 3.0.0.

## Debugger

* A new debugger [debug.gem](https://github.com/ruby/debug) is bundled.
  debug.gem is a fast debugger implementation, and it provides many features
  like remote debugging, colorful REPL, IDE (VSCode) integration, and more.
  It replaces `lib/debug.rb` standard library.

* `rdbg` command is also installed into `bin/` directory to start and control
  debugging execution.

## error_highlight

A built-in gem called error_highlight has been introduced.
It shows fine-grained error locations in the backtrace.

Example: `title = json[:article][:title]`

If `json` is nil, it shows:

```
$ ruby test.rb
test.rb:2:in `<main>': undefined method `[]' for nil:NilClass (NoMethodError)

title = json[:article][:title]
            ^^^^^^^^^^
```

If `json[:article]` returns nil, it shows:

```
$ ruby test.rb
test.rb:2:in `<main>': undefined method `[]' for nil:NilClass (NoMethodError)

title = json[:article][:title]
                      ^^^^^^^^
```

This feature is enabled by default.
You can disable it by using a command-line option `--disable-error_highlight`.
See [the repository](https://github.com/ruby/error_highlight) in detail.

## Miscellaneous changes

* lib/objspace/trace.rb is added, which is a tool for tracing the object
  allocation. Just by requiring this file, tracing is started *immediately*.
  Just by `Kernel#p`, you can investigate where an object was created.
  Note that just requiring this file brings a large performance overhead.
  This is only for debugging purposes. Do not use this in production.
  [[Feature #17762]]

* Now exceptions raised in finalizers will be printed to `STDERR`, unless
  `$VERBOSE` is `nil`.  [[Feature #17798]]

* `ruby -run -e httpd` displays URLs to access.  [[Feature #17847]]

* Add `ruby -run -e colorize` to colorize Ruby code using
  `IRB::Color.colorize_code`.

[Bug #4443]:      https://bugs.ruby-lang.org/issues/4443
[Feature #6210]:  https://bugs.ruby-lang.org/issues/6210
[Feature #11256]: https://bugs.ruby-lang.org/issues/11256
[Feature #11689]: https://bugs.ruby-lang.org/issues/11689
[Feature #12194]: https://bugs.ruby-lang.org/issues/12194
[Feature #12495]: https://bugs.ruby-lang.org/issues/12495
[Feature #12913]: https://bugs.ruby-lang.org/issues/12913
[Feature #14256]: https://bugs.ruby-lang.org/issues/14256
[Feature #14394]: https://bugs.ruby-lang.org/issues/14394
[Feature #14579]: https://bugs.ruby-lang.org/issues/14579
[Feature #15198]: https://bugs.ruby-lang.org/issues/15198
[Feature #15211]: https://bugs.ruby-lang.org/issues/15211
[Feature #15912]: https://bugs.ruby-lang.org/issues/15912
[Feature #16043]: https://bugs.ruby-lang.org/issues/16043
[Feature #16182]: https://bugs.ruby-lang.org/issues/16182
[Feature #16806]: https://bugs.ruby-lang.org/issues/16806
[Feature #17312]: https://bugs.ruby-lang.org/issues/17312
[Feature #17327]: https://bugs.ruby-lang.org/issues/17327
[Feature #17370]: https://bugs.ruby-lang.org/issues/17370
[Feature #17398]: https://bugs.ruby-lang.org/issues/17398
[Feature #17411]: https://bugs.ruby-lang.org/issues/17411
[Bug #17423]:     https://bugs.ruby-lang.org/issues/17423
[Bug #17429]:     https://bugs.ruby-lang.org/issues/17429
[Feature #17470]: https://bugs.ruby-lang.org/issues/17470
[Feature #17479]: https://bugs.ruby-lang.org/issues/17479
[Feature #17485]: https://bugs.ruby-lang.org/issues/17485
[Feature #17544]: https://bugs.ruby-lang.org/issues/17544
[Feature #17592]: https://bugs.ruby-lang.org/issues/17592
[Feature #17684]: https://bugs.ruby-lang.org/issues/17684
[Feature #17724]: https://bugs.ruby-lang.org/issues/17724
[Feature #17744]: https://bugs.ruby-lang.org/issues/17744
[Feature #17750]: https://bugs.ruby-lang.org/issues/17750
[Feature #17762]: https://bugs.ruby-lang.org/issues/17762
[Feature #17795]: https://bugs.ruby-lang.org/issues/17795
[Feature #17798]: https://bugs.ruby-lang.org/issues/17798
[Bug #17827]:     https://bugs.ruby-lang.org/issues/17827
[Feature #17847]: https://bugs.ruby-lang.org/issues/17847
[Feature #17853]: https://bugs.ruby-lang.org/issues/17853
[Bug #18003]:     https://bugs.ruby-lang.org/issues/18003
[Feature #18008]: https://bugs.ruby-lang.org/issues/18008
[Feature #18015]: https://bugs.ruby-lang.org/issues/18015
[Feature #18020]: https://bugs.ruby-lang.org/issues/18020
[Feature #18029]: https://bugs.ruby-lang.org/issues/18029
[Feature #18045]: https://bugs.ruby-lang.org/issues/18045
[Feature #18148]: https://bugs.ruby-lang.org/issues/18148
[Feature #18172]: https://bugs.ruby-lang.org/issues/18172
[Feature #18176]: https://bugs.ruby-lang.org/issues/18176
[Feature #18190]: https://bugs.ruby-lang.org/issues/18190
[Feature #18229]: https://bugs.ruby-lang.org/issues/18229
[Feature #18239]: https://bugs.ruby-lang.org/issues/18239
[Feature #18254]: https://bugs.ruby-lang.org/issues/18254
[Feature #18273]: https://bugs.ruby-lang.org/issues/18273
[Feature #18290]: https://bugs.ruby-lang.org/issues/18290

[GH-1509]: https://github.com/ruby/ruby/pull/1509
[GH-4815]: https://github.com/ruby/ruby/pull/4815
[GH-5112]: https://github.com/ruby/ruby/pull/5112
[GH-5131]: https://github.com/ruby/ruby/pull/5131
[GH-5146]: https://github.com/ruby/ruby/pull/5146