Write specs for new Ruby 2.7 features and changes

[eregon]

ruby/spec already contains some specs for 2.7, but we should aim to cover all new features and important changes.
This will improve the test coverage of these features (and maybe discover a few bugs along the way), allow other Ruby implementations to implement the changes faster with more confidence and document clearly the new behavior.

The new specs should be within a version guard block:

ruby_version_is "2.7" do
  # New specs
end

NOTE: https://rubyreferences.github.io/rubychanges/2.7.html gives more details for many features and changes.

From https://github.com/ruby/ruby/blob/v2_7_0/NEWS:

NEWS for Ruby 2.7.0

This document is a list of user visible feature changes made between
releases 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).

Changes since the 2.6.0 release

Language changes

Pattern matching

• Pattern matching is introduced as an experimental feature. Feature #14912

case [0, [1, 2, 3]]
in [a, [b, *c]]
  p a #=> 0
  p b #=> 1
  p c #=> [2, 3]
end

case {a: 0, b: 1}
in {a: 0, x: 1}
  :unreachable
in {a: 0, b: var}
  p var #=> 1
end

case -1
in 0 then :unreachable
in 1 then :unreachable
end #=> NoMatchingPatternError

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

JSON.parse(json, symbolize_names: true) in {name: "Alice", children: [{name: name, age: age}]}

p name #=> "Bob"
p age  #=> 2

JSON.parse(json, symbolize_names: true) in {name: "Alice", children: [{name: "Charlie", age: age}]}
#=> NoMatchingPatternError

See the following slides for more details:

• https://speakerdeck.com/k_tsj/pattern-matching-new-feature-in-ruby-2-dot-7
• Note that the slides are slightly obsolete.

• The warning against pattern matching can be suppressed with
  {-W:no-experimental option}[#label-Warning+option].

The spec of keyword arguments is changed towards 3.0

• Non-symbols are allowed as keyword argument keys if the method accepts
  arbitrary keywords. Feature #14183

  Non-Symbol keys in a keyword arguments hash were prohibited in 2.6.0,
  but are now allowed again. Bug #15658

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

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

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)    # {}

• Above warnings can be suppressed also with {-W:no-deprecated option}[#label-Warning+option].

Numbered parameters

• Numbered parameters as default block parameters are introduced.
  Feature #4475

  [1, 2, 10].map { _1.to_s(16) }    #=> ["1", "2", "a"]
  [[1, 2], [3, 4]].map { _1 + _2 }  #=> [3, 7]

  You can still define a local variable named _1 and so on,
  and that is honored when present, but renders a warning.

  _1 = 0            #=> warning: `_1' is reserved for numbered parameter; consider another name
  [1].each { p _1 } # prints 0 instead of 1

proc/lambda without block is deprecated

• Proc.new and Kernel#proc with no block in a method called with a block is
  warned now.

  def foo
    proc
  end
  foo { puts "Hello" } #=> warning: Capturing the given block using Kernel#proc is deprecated; use `&block` instead

  This warning can be suppressed with {-W:no-deprecated option}[#label-Warning+option].

• Kernel#lambda with no block in a method called with a block raises an exception.

  def bar
    lambda
  end
  bar { puts "Hello" } #=> tried to create Proc object without a block (ArgumentError)

Other miscellaneous changes

• A beginless range is experimentally introduced. It might be useful
  in case, new call-sequence of the Comparable#clamp,
  constants and DSLs. Feature #14799

  ary[..3]  # identical to ary[0..3]
  
  case RUBY_VERSION
  when ..."2.4" then puts "EOL"
  # ...
  end
  
  age.clamp(..100)
  
  where(sales: ..100)

• Quoted here-document identifiers must end within the same line.

  <<"EOS
  " # This had been warned since 2.4; Now it raises a SyntaxError
  EOS
  

• The flip-flop syntax deprecation is reverted. Feature #5400

• Comment lines can be placed between fluent dot now.

  foo
    # .bar
    .baz # => foo.baz

• Calling a private method with a literal self as the receiver
  is now allowed. Feature #11297 Feature #16123

• Modifier rescue now operates the same for multiple assignment as single
  assignment. Bug #8279

  a, b = raise rescue [1, 2]
  # Previously parsed as: (a, b = raise) rescue [1, 2]
  # Now parsed as:         a, b = (raise rescue [1, 2])

• yield in singleton class syntax is warned and will be deprecated later. Feature #15575.

  def foo
    class << Object.new
      yield #=> warning: `yield' in class syntax will not be supported from Ruby 3.0. [Feature #15575](https://bugs.ruby-lang.org/issues/15575)
    end
  end
  foo { p :ok }

  This warning can be suppressed with {-W:no-deprecated option}[#label-Warning+option].

• Argument forwarding by (...) is introduced. Feature #16253

  def foo(...)
    bar(...)
  end

  All arguments to foo are forwarded to bar, including keyword and
  block arguments.
  Note that the parentheses are mandatory. bar ... is parsed
  as an endless range.

• Access and setting of $SAFE is now always warned. $SAFE
  will become a normal global variable in Ruby 3.0. Feature #16131

• Object#{taint,untaint,trust,untrust} and related functions in the C-API
  no longer have an effect (all objects are always considered untainted), and are now
  warned in verbose mode. This warning will be disabled even in non-verbose mode in
  Ruby 3.0, and the methods and C functions will be removed in Ruby 3.2. Feature #16131

• Refinements take place at Object#method and Module#instance_method. Feature #15373

Command line options

Warning option

The -W option has been extended with a following :, to manage categorized
warnings. Feature #16345 Feature #16420

• To suppress deprecation warnings:

  $ ruby -e '$; = ""'
  -e:1: warning: `$;' is deprecated
  
  $ ruby -W:no-deprecated -e '$; = //'
  

• It works with the RUBYOPT environment variable:

  $ RUBYOPT=-W:no-deprecated ruby -e '$; = //'
  

• To suppress experimental feature warnings:

  $ ruby -e '0 in a'
  -e:1: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
  
  $ ruby -W:no-experimental -e '0 in a'
  

• To suppress both by using RUBYOPT, set space separated values:

  $ RUBYOPT='-W:no-deprecated -W:no-experimental' ruby -e '($; = "") in a'
  

See also Warning in {Core classes updates}[#label-Core+classes+updates+-28outstanding+ones+only-29].

Core classes updates (outstanding ones only)

Array

• Added Array#intersection. Feature #16155

• Added Array#minmax, with a faster implementation than Enumerable#minmax. Bug #15929

Comparable

• Comparable#clamp now accepts a Range argument. Feature #14784

    ```ruby
    -1.clamp(0..2) #=> 0
     1.clamp(0..2) #=> 1
     3.clamp(0..2) #=> 2
    # With beginless and endless ranges:
    -1.clamp(0..)  #=> 0
     3.clamp(..2)  #=> 2
    ```
  

Complex

• Added Complex#<=>.
  So 0 <=> 0i will not raise NoMethodError. Bug #15857

Dir

• Dir.glob and Dir.[] no longer allow NUL-separated glob pattern.
  Use Array instead. Feature #14643

Encoding

• Added new encoding CESU-8. Feature #15931

Enumerable

• Added Enumerable#filter_map. Feature #15323

    [1, 2, 3].filter_map {|x| x.odd? ? x.to_s : nil } #=> ["1", "3"]
  

• Added Enumerable#tally. Feature #11076

    ["A", "B", "C", "B", "A"].tally #=> {"A"=>2, "B"=>2, "C"=>1}
  

Enumerator

• Added Enumerator.produce to generate an Enumerator from any custom
  data transformation. Feature #14781

    require "date"
    dates = Enumerator.produce(Date.today, &:succ) #=> infinite sequence of dates
    dates.detect(&:tuesday?) #=> next Tuesday
  

• Added Enumerator::Lazy#eager that generates a non-lazy enumerator
  from a lazy enumerator. Feature #15901

    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!?"]
  

• Added Enumerator::Yielder#to_proc so that a Yielder object
  can be directly passed to another method as a block
  argument. Feature #15618

Fiber

• Added Fiber#raise that behaves like Fiber#resume but raises an
  exception on the resumed fiber. Feature #10344

File

• File.extname now returns a dot string for names ending with a dot on
  non-Windows platforms. Bug #15267

      File.extname("foo.") #=> "."
  

FrozenError

• Added FrozenError#receiver to return the frozen object on which
  modification was attempted. To set this object when raising
  FrozenError in Ruby code, FrozenError.new accepts a :receiver
  option. Feature #15751

GC

• (MRI-specific) Added GC.compact method 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 (copy-on-write) friendly. Feature #15626

  Details on the algorithm and caveats can be found here:
  https://bugs.ruby-lang.org/issues/15626
  

IO

• Added IO#set_encoding_by_bom to check the BOM and set the external
  encoding. Bug #15210

Integer

• Integer#[] now supports range operations. Feature #8842

     0b01001101[2, 4]  #=> 0b0011
     0b01001100[2..5]  #=> 0b0011
     0b01001100[2...6] #=> 0b0011
     #   ^^^^
  

Method

• Method#inspect shows more information. Feature #14145

Module

• Added Module#const_source_location to retrieve the location where a
  constant is defined. Feature #10771

• Added Module#ruby2_keywords for marking a method as passing keyword
  arguments through a regular argument splat, useful when delegating
  all arguments to another method in a way that can be backwards
  compatible with older Ruby versions. Bug #16154

• Module#autoload? now takes an inherit optional argument, like
  Module#const_defined?. Feature #15777

• Module#name now always returns a frozen String. The returned String is
  always the same for a given Module. This change is
  experimental. Feature #16150

NilClass / TrueClass / FalseClass

• NilClass#to_s, TrueClass#to_s, and FalseClass#to_s now always return a
  frozen String. The returned String is always the same for each of these
  values. This change is experimental. Feature #16150

ObjectSpace::WeakMap

• ObjectSpace::WeakMap#[]= now accepts special objects as either key or
  values. Feature #16035

Proc

• Added Proc#ruby2_keywords for marking the proc as passing keyword
  arguments through a regular argument splat, useful when delegating
  all arguments to another method or proc in a way that can be backwards
  compatible with older Ruby versions. Feature #16404

Range

• Added Range#minmax, with a faster implementation than Enumerable#minmax.
  It returns a maximum that now corresponds to Range#max. Bug #15807

• Range#=== now uses Range#cover? for String arguments, too (in Ruby 2.6, it was
  changed from Range#include? for all types except strings). Bug #15449

String

• Update Unicode version and Emoji version from 11.0.0 to
  12.0.0. Feature #15321

• Update Unicode version to 12.1.0, adding support for
  U+32FF SQUARE ERA NAME REIWA. Feature #15195

• Update Unicode Emoji version to 12.1. Feature #16272

Symbol

• Added Symbol#start_with? and Symbol#end_with? methods. Feature #16348

Time

• Added Time#ceil method. Feature #15772

• Added Time#floor method. Feature #15653

• Time#inspect is separated from Time#to_s and it shows
  the time's sub second. Feature #15958

UnboundMethod

• Added UnboundMethod#bind_call method. Feature #15955

  <code>umethod.bind_call(obj, ...)</code> is semantically equivalent
  to <code>umethod.bind(obj).call(...)</code>.  This idiom is used in
  some libraries to call a method that is overridden.  The added
  method does the same without allocation of an intermediate Method
  object.
  
      class Foo
        def add_1(x)
          x + 1
        end
      end
      class Bar < Foo
        def add_1(x) # override
          x + 2
        end
      end
  
      obj = Bar.new
      p obj.add_1(1) #=> 3
      p Foo.instance_method(:add_1).bind(obj).call(1) #=> 2
      p Foo.instance_method(:add_1).bind_call(obj, 1) #=> 2
  

Warning

• Added Warning.[] and Warning.[]= to manage emitting/suppressing
  some categories of warnings. Feature #16345 Feature #16420

$LOAD_PATH

• Added $LOAD_PATH.resolve_feature_path. Feature #15903 Feature #15230

Stdlib updates (outstanding ones only)

• Pathname.glob now delegates 3 arguments to Dir.glob
  to accept base keyword. Feature #14405

Compatibility issues (excluding feature bug fixes)

• The following libraries are no longer bundled gems.
  Install corresponding gems to use these features.
  • CMath (cmath gem)
  • Scanf (scanf gem)
  • Shell (shell gem)
  • Synchronizer (sync gem)
  • ThreadsWait (thwait gem)
  • E2MM (e2mmap gem)

Proc

• The Proc#to_s format was changed. Feature #16101

Range

• Add Range#minmax specs #777 Range#minmax used to iterate on the range to determine the maximum.
  It now uses the same algorithm as Range#max. In rare cases (e.g.
  ranges of Floats or Strings), this may yield different results. Bug #15807

Stdlib compatibility issues (excluding feature bug fixes)

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

• The did_you_mean gem has been promoted up to a default gem from a bundled gem

pathname

• Kernel#Pathname when called with a Pathname argument now returns
  the argument instead of creating a new Pathname. This is more
  similar to other Kernel methods, but can break code that modifies
  the return value and expects the argument not to be modified.

C API updates

• Many *_kw functions have been added for setting whether
  the final argument being passed should be treated as keywords. You
  may need to switch to these functions to avoid keyword argument
  separation warnings, and to ensure correct behavior in Ruby 3.

• The : character in rb_scan_args format string is now
  treated as keyword arguments. Passing a positional hash instead of
  keyword arguments will emit a deprecation warning.

• C API declarations with ANYARGS are changed not to use ANYARGS.
  See Delete ANYARGS ruby#2404

[eregon]

@andrykonchin I tried to keep the ones you checked after my last fix for links to issues. Should be fine but would be good if you can double-check.

[andrykonchin]

@eregon Checked 👍

[eregon]

Note for contributors: please mention here which item/checkbox you'd like to add specs for, in order to avoid duplicated work.

[lxxxvi]

Hi @eregon

I believe you can mark this as completed

• "Added Symbol#start_with? and Symbol#end_with? methods."

Symbol#end_with?: c51c102

Symbol#start_with?: 7377c2b

[moofkit]

And this looks like completed too

The Proc#to_s format was changed. Feature #16101

d3c1244

[lxxxvi]

@eregon

How could I cover this one?

• Added Array#minmax, with a faster implementation than Enumerable#minmax. Bug #15929

As far as I understand there's already a Enumerable#minmax spec which uses Arrays... so it's already tested?