# 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 arguments can be 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.

*   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, 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]]

## Command line options

## Core classes updates

Outstanding ones only.

* 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-soconds.

    * 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]]

* MatchData

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

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

* 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 overwride this method to
      hook fork event. [[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]]

* 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]]

* $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
    * Bundler
    * RDoc 6.3.2
    * ReLine
    * JSON 2.6.1
    * Psych 4.0.2
    * FileUtils 1.6.0
    * Fiddle 1.1.0
    * StringIO 3.0.1
    * IO::Console 0.5.9
    * IO::Wait 0.2.0
    * CSV 3.2.1
    * Etc 1.3.0
    * Date 3.2.2
    * Zlib 2.1.1
    * StringScanner 3.0.1
    * IpAddr 1.2.3
    * Logger 1.4.4
    * OStruct 0.5.0
    * Irb
    * Racc 1.6.0
    * Delegate 0.2.0
    * Benchmark 0.2.0
    * CGI 0.3.1
    * Readline(C-ext) 0.1.3
    * Timeout 0.2.0
    * YAML 0.2.0
    * URI 0.11.0
    * OpenSSL
    * DidYouMean
    * Weakref 0.1.1
    * Tempfile 0.1.2
    * TmpDir 0.1.2
    * English 0.7.1
    * Net::Protocol 0.1.2
    * Net::Http 0.2.0
    * BigDecimal
    * OptionParser 0.2.0
    * Set 1.0.2
    * Find 0.1.1
    * Rinda 0.1.1
    * Erb 2.2.3
    * NKF 0.1.1
    * Base64 0.1.1
    * OpenUri 0.2.0
    * SecureRandom 0.1.1
    * Resolv 0.2.1
    * Resolv::Replace 0.1.0
    * Time 0.2.0
    * PP 0.2.1
    * Prettyprint 0.1.1
    * Drb 2.1.0
    * Pathname 0.2.0
    * Digest 3.1.0.pre3
    * Un 0.2.0
*   The following bundled gems are updated.
    * minitest 5.14.4
    * power_assert 2.0.1
    * rake 13.0.6
    * test-unit 3.5.1
    * rbs 1.8.0
    * typeprof 0.20.4
*   The following default gems are now bundled gems.
    * net-ftp
    * net-imap
    * net-pop
    * net-smtp
    * matrix
    * prime

## Compatibility issues

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]]

### JIT

* The default `--jit-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.

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

* `RubyVM::MJIT` is renamed to `RubyVM::JIT`. [[Feature #17490]]

### 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

### 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 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, error_highlight, has been introduced.
It includes fine-grained error location in backtrace:

```
$ ruby test.rb
test.rb:1:in `<main>': undefined method `time' for 1:Integer (NoMethodError)

1.time {}
 ^^^^^
Did you mean?  times
```

This gem 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 purpose. Do not use this in production.
  [[Feature #17762]]

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

[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 #12194]: https://bugs.ruby-lang.org/issues/12194
[Feature #12495]: https://bugs.ruby-lang.org/issues/12495
[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 #16043]: https://bugs.ruby-lang.org/issues/16043
[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 #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 #17490]: https://bugs.ruby-lang.org/issues/17490
[Feature #17592]: https://bugs.ruby-lang.org/issues/17592
[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 #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 #18172]: https://bugs.ruby-lang.org/issues/18172
[Feature #18229]: https://bugs.ruby-lang.org/issues/18229
[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