ruby/spec already contains some specs for 3.1, 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.

Please take a look at open pull requests before starting on a task to avoid duplicated work.

The new specs should be within a version guard block:

ruby_version_is "3.1" do
  # New specs
end

NOTE: https://rubyreferences.github.io/rubychanges/3.1.html gives more details for many features and changes.

From https://github.com/ruby/ruby/blob/master/doc/NEWS/NEWS-3.1.0.md:

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

• bcbc45e The block argument can now be anonymous if the block will
  only be passed to another method. [Feature #11256]

  def foo(&)
    bar(&)
  end

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

  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]

  @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]

  [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:

  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:

  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]

Core classes updates

Note: We're only listing outstanding class updates.

Array

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

Class

• 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. [[Feature #10917]]

• GC.total_time returns measured time in nano-seconds. [[Feature #10917]]

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]

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]

Thread::Queue

• Thread::Queue.new now accepts an Enumerable of initial values.
  [Feature #17327]

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]

    ```ruby
    Time.new(2021, 12, 25, in: "+07:00")
    #=> 2021-12-25 00:00:00 +0700
    ```
  
    At the same time, time component strings are converted to
    integers more strictly now.
  
    ```ruby
    Time.new(2021, 12, 25, "+07:30")
    #=> invalid value for Integer(): "+07:30" (ArgumentError)
    ```
  
    Ruby 3.0 or earlier returned probably unexpected result
    `2021-12-25 07:00:00`, not `2021-12-25 07:30:00` nor
    `2021-12-25 00:00:00 +07:30`.
  

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

TracePoint

• TracePoint.allow_reentry 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 3.3.3
  • base64 0.1.1
  • benchmark 0.2.0
  • bigdecimal 3.1.1
  • bundler 2.3.3
  • cgi 0.3.1
  • csv 3.2.2
  • date 3.2.2
  • did_you_mean 1.6.1
  • digest 3.1.0
  • drb 2.1.0
  • erb 2.2.3
  • error_highlight 0.3.0
  • etc 1.3.0
  • fcntl 1.0.1
  • fiddle 1.1.0
  • fileutils 1.6.0
  • find 0.1.1
  • io-console 0.5.10
  • io-wait 0.2.1
  • ipaddr 1.2.3
  • irb 1.4.1
  • json 2.6.1
  • logger 1.5.0
  • net-http 0.2.0
  • net-protocol 0.1.2
  • nkf 0.1.1
  • open-uri 0.2.0
  • openssl 3.0.0
  • optparse 0.2.0
  • ostruct 0.5.2
  • pathname 0.2.0
  • pp 0.3.0
  • prettyprint 0.1.1
  • psych 4.0.3
  • racc 1.6.0
  • rdoc 6.4.0
  • readline 0.0.3
  • readline-ext 0.1.4
  • reline 0.3.0
  • 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 2.0.0
  • typeprof 0.21.1
• 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]

• Psych 4.0 changes Psych.load as safe_load by the default.
  You may need to use Psych 3.3.2 for migrating to this behavior.
  [Bug #17866]

C API updates

• Documented. [GH-4815]

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

Implementation improvements

• Inline cache mechanism is introduced for reading class variables.
  [Feature #17763]

• 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]

Debugger

• A new debugger debug.gem 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 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]

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