NEWS for Ruby 3.0.0

This document is a list of user visible feature changes since the 2.7.0 release, 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. https://bugs.ruby-lang.org/issues/$FEATURE_OR_BUG_NUMBER).

Language changes

Keyword arguments are now separated from positional arguments. Code that resulted in deprecation warnings in Ruby 2.7 will now result in ArgumentError or different behavior. [Feature #14183]
Arguments forwarding (...) now supports leading arguments. [Feature #16378]
```
def method_missing(meth, ...)
  send(:"do_#{meth}", ...)
end
```

Procs accepting a single rest argument and keywords are no longer subject to autosplatting. This now matches the behavior of Procs accepting a single rest argument and no keywords. [Feature #16166]

pr = proc{|*a, **kw| [a, kw]}

pr.call([1])
# 2.7 => [[1], {}]
# 3.0 => [[[1]], {}]

pr.call([1, {a: 1}])
# 2.7 => [[1], {:a=>1}] # and deprecation warning
# 3.0 => [[[1, {:a=>1}]], {}]

$SAFE is now a normal global variable with no special behavior. C-API methods related to $SAFE have been removed. [Feature #16131]
yield in singleton class definitions in methods is now a SyntaxError instead of a warning. yield in a class definition outside of a method is now a SyntaxError instead of a LocalJumpError. [Feature #15575]

Find pattern is added. [Feature #16828]

case ["a", 1, "b", "c", 2, "d", "e", "f", 3]
in [*pre, String => x, String => y, *post]
  p pre  #=> ["a", 1]
  p x    #=> "b"
  p y    #=> "c"
  p post #=> [2, "d", "e", "f", 3]
end

When a class variable is overtaken by the same definition in an ancestor class/module, a RuntimeError is now raised (previously, it only issued a warning in verbose mode. Additionally, accessing a class variable from the toplevel scope is now a RuntimeError. [Bug #14541]
Rightward assignment statement is added. [EXPERIMENTAL] [Feature #15921]
```
fib(10) => x
```
Endless method definition is added. [EXPERIMENTAL] [Feature #16746]
```
def square(x) = x * x
```

Command line options

`--help` option

When the environment variable RUBY_PAGER or PAGER is present and has non-empty value, and the standard input and output are tty, --help option shows the help message via the pager designated by the value. [Feature #16754]

Core classes updates

Outstanding ones only.

Dir
- Modified method
  - Dir.glob and Dir.[] now sort the results by default, and accept sort: keyword option. [Feature #8709]
ENV
- New method
  - ENV.except, which returns a hash excluding the given keys and their values. [Feature #15822]
Hash
- Modified method
  - Hash#transform_keys now accepts a hash that maps keys to new keys. [Feature #16274]
- New method
  - Hash#except, which returns a hash excluding the given keys and their values. [Feature #15822]
Kernel
- Modified method
  - Kernel#clone when called with freeze: false keyword will call #initialize_clone with the freeze: false keyword. [Bug #14266]
  - Kernel#clone when called with freeze: true keyword will call #initialize_clone with the freeze: true keyword, and will return a frozen copy even if the receiver is unfrozen. [Feature #16175]
  - Kernel#eval when called with two arguments will use "(eval)" for __FILE__ and 1 for __LINE__ in the evaluated code. [Bug #4352]
  - Kernel#lambda now warns if called without a literal block. [Feature #15973]
Module
- Modified method
  - Module#include and #prepend now affect classes and modules that have already included or prepended the receiver, mirroring the behavior if the arguments were included in the receiver before the other modules and classes included or prepended the receiver. [Feature #9573]
```
class C; end
module M1; end
module M2; end
C.include M1
M1.include M2
p C.ancestors #=> [C, M1, M2, Object, Kernel, BasicObject]
```
Symbol
- Modified method
  - Symbol#to_proc now returns a lambda Proc. [Feature #16260]

Stdlib updates

Outstanding ones only.

RubyGems
- Update to RubyGems 3.2.0.pre1
Bundler
- Update to Bundler 2.2.0.dev
Net::HTTP
- New method
  - Add Net::HTTP#verify_hostname= and Net::HTTP#verify_hostname to skip hostname verification. [Feature #16555]
- Modified method
  - Net::HTTP.get, Net::HTTP.get_response, and Net::HTTP.get_print can take request headers as a Hash in the second argument when the first argument is a URI. [Feature #16686]
Tempfile
- Modified method
  - Tempfile.open { ... } will now unlink the file at the end of the block (https://github.com/ruby/tempfile/pull/3), such that once the block finishes execution nothing leaks.

Compatibility issues

Excluding feature bug fixes.

Regexp literals are frozen [Feature #8948] [Feature #16377]
```
/foo/.frozen? #=> true
```
EXPERIMENTAL: Hash#each consistently yields a 2-element array [Bug #12706]
- Now { a: 1 }.each(&->(k, v) { }) raises an ArgumentError due to lambda's arity check.
- This is experimental; if it brings a big incompatibility issue, it may be reverted until 2.8/3.0 release.
When writing to STDOUT redirected to a closed pipe, no broken pipe error message will be shown now. [Feature #14413]
TRUE/FALSE/NIL constants are no longer defined.
Integer#zero? overrides Numeric#zero? for optimization. [Misc #16961]

Stdlib compatibility issues

Default gems
- The following libraries are promoted the default gems from stdlib.
  - English
  - erb
  - find
  - io-nonblock
  - io-wait
  - net-ftp
  - net-http
  - net-imap
  - net-protocol
  - optparse
  - rinda
  - set
  - tempfile
  - tmpdir
  - weakref
Bundled gems
- net-telnet and xmlrpc have been removed from the bundled gems. If you are interested in maintaining them, please comment on your plan to https://github.com/ruby/xmlrpc or https://github.com/ruby/net-telnet.
SDBM have been removed from ruby standard library. [Bug #8446]
- The issues of sdbm will be handled at https://github.com/ruby/sdbm

C API updates

C API functions related to $SAFE have been removed. [Feature #16131]
C API header file ruby/ruby.h was split. [GH-2991] Should have no impact on extension libraries, but users might experience slow compilations.

Implementation improvements

New method cache mechanism for Ractor [Feature #16614]
- TODO: ko1 will write details
The number of hashes allocated when using a keyword splat in a method call has been reduced to a maximum of 1, and passing a keyword splat to a method that accepts specific keywords does not allocate a hash.
super is optimized when the same type of method is called in the previous call if it's not refinements or an attr reader or writer.

JIT

Native functions shared by multiple methods are deduplicated on JIT compaction.
Decrease code size of hot paths by some optimizations and partitioning cold paths.
Not only pure Ruby methods but also some C methods skip pushing a method frame.
- Kernel#class, Integer#zero?
Always generate appropriate code for ==, nil?, and ! calls depending on a receiver class.
Optimize instance variable access in some core classes like Hash and their subclasses
Eliminate VM register access on a method return
Optimize C method call a little

Miscellaneous changes

Methods using ruby2_keywords will no longer keep empty keyword splats, those are now removed just as they are for methods not using ruby2_keywords.
Taint deprecation warnings are now issued in regular mode in addition to verbose warning mode. [Feature #16131]
When an exception is caught in the default handler, the error message and backtrace are printed in order from the innermost. [Feature #8661]

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