Support TargetRubyVersion 3.1 (experimental)

RuboCop AST 1.5 supports Ruby 3.1.
rubocop/rubocop-ast#182.

This PR supports `TargetRubyVersion 3.1`.

changelog/new_support_ruby_3_1_experimental.md

@@ -0,0 +1 @@
+* [#9758](https://github.com/rubocop/rubocop/pull/9758): Support `TargetRubyVersion 3.1` (experimental). ([@koic][])

docs/modules/ROOT/pages/compatibility.adoc

@@ -31,6 +31,7 @@ The following table is the support matrix.
 | 2.6 | -
 | 2.7 | -
 | 3.0 | -
+| 3.1 (experimental) | -
 |===
 
 NOTE: The compatibility xref:configuration.adoc#setting-the-target-ruby-version[target Ruby version mentioned here] is about code analysis (what RuboCop can analyze), not runtime (is RuboCop capable of running on some Ruby or not).

lib/rubocop/rspec/shared_contexts.rb

@@ -131,3 +131,7 @@ def source_range(range, buffer: source_buffer)
 RSpec.shared_context 'ruby 3.0', :ruby30 do
   let(:ruby_version) { 3.0 }
 end
+
+RSpec.shared_context 'ruby 3.1', :ruby31 do
+  let(:ruby_version) { 3.1 }
+end

lib/rubocop/target_ruby.rb

@@ -4,7 +4,7 @@ module RuboCop
   # The kind of Ruby that code inspected by RuboCop is written in.
   # @api private
   class TargetRuby
-    KNOWN_RUBIES = [2.5, 2.6, 2.7, 3.0].freeze
+    KNOWN_RUBIES = [2.5, 2.6, 2.7, 3.0, 3.1].freeze
     DEFAULT_VERSION = KNOWN_RUBIES.first
 
     OBSOLETE_RUBIES = {

rubocop.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
   s.add_runtime_dependency('regexp_parser', '>= 1.8', '< 3.0')
   s.add_runtime_dependency('rexml')
-  s.add_runtime_dependency('rubocop-ast', '>= 1.2.0', '< 2.0')
+  s.add_runtime_dependency('rubocop-ast', '>= 1.5.0', '< 2.0')
   s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 3.0')
 

spec/rubocop/cli_spec.rb

@@ -1612,7 +1612,7 @@ def method(foo, bar, qux, fred, arg5, f) end #{'#' * 85}
         expect($stderr.string.strip).to start_with(
           'Error: RuboCop found unknown Ruby version 4.0 in `TargetRubyVersion`'
         )
-        expect($stderr.string.strip).to match(/Supported versions: 2.5, 2.6, 2.7, 3.0/)
+        expect($stderr.string.strip).to match(/Supported versions: 2.5, 2.6, 2.7, 3.0, 3.1/)
       end
     end