Permit to specify TargetRubyVersion 2.8

This PR permits to specify TargetRubyVersion 2.8.

RuboCop AST supports Ruby 2.8 (3.0) with
rubocop/rubocop-ast#48.

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### New features
 
 * [#7868](https://github.com/rubocop-hq/rubocop/pull/7868): `Cop::Base` is the new recommended base class for cops. ([@marcandre][])
+* [#8213](https://github.com/rubocop-hq/rubocop/pull/8213): Permit to specify TargetRubyVersion 2.8 (experimental). ([@koic][])
 
 ### Changes
 

docs/modules/ROOT/pages/compatibility.adoc

@@ -30,6 +30,7 @@ The following table is the support matrix.
 | 2.5 | -
 | 2.6 | -
 | 2.7 | -
+| 2.8 (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/target_ruby.rb

@@ -3,7 +3,7 @@
 module RuboCop
   # The kind of Ruby that code inspected by RuboCop is written in.
   class TargetRuby
-    KNOWN_RUBIES = [2.4, 2.5, 2.6, 2.7].freeze
+    KNOWN_RUBIES = [2.4, 2.5, 2.6, 2.7, 2.8].freeze
     DEFAULT_VERSION = KNOWN_RUBIES.first
 
     OBSOLETE_RUBIES = {

rubocop.gemspec

@@ -38,7 +38,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
   s.add_runtime_dependency('regexp_parser', '>= 1.7')
   s.add_runtime_dependency('rexml')
-  s.add_runtime_dependency('rubocop-ast', '>= 0.0.3', '< 1.0')
+  s.add_runtime_dependency('rubocop-ast', '>= 0.1.0', '< 1.0')
   s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 2.0')
 

spec/rubocop/cli_spec.rb

@@ -1632,14 +1632,14 @@ def method(foo, bar, qux, fred, arg5, f) end #{'#' * 85}
       it 'fails with an error message' do
         create_file('.rubocop.yml', <<~YAML)
           AllCops:
-            TargetRubyVersion: 2.8
+            TargetRubyVersion: 3.0
         YAML
         expect(cli.run([])).to eq(2)
         expect($stderr.string.strip).to start_with(
-          'Error: RuboCop found unknown Ruby version 2.8 in `TargetRubyVersion`'
+          'Error: RuboCop found unknown Ruby version 3.0 in `TargetRubyVersion`'
         )
         expect($stderr.string.strip).to match(
-          /Supported versions: 2.4, 2.5, 2.6, 2.7/
+          /Supported versions: 2.4, 2.5, 2.6, 2.7, 2.8/
         )
       end
     end