Support Parser::Ruby28

Parser gem has been started development for Ruby 2.8 (edge Ruby).
whitequark/parser#677

This PR supports `Parser::Ruby28`, the early adapters will be able to
try edge Ruby with RuboCop.

I will open a PR later to new Ruby 2.8 (3.0) syntax supported by Parser.

CHANGELOG.md

@@ -12,6 +12,7 @@
 * [#31](https://github.com/rubocop-hq/rubocop-ast/pull/31): NodePattern now uses `param === node` to match params, which allows Regexp, Proc, Set in addition to Nodes and literals. ([@marcandre][])
 * [#41](https://github.com/rubocop-hq/rubocop-ast/pull/41): Add `delimiters` and related predicates for `RegexpNode`. ([@owst][])
 * [#46](https://github.com/rubocop-hq/rubocop-ast/pull/46): Basic support for [non-legacy AST output from parser](https://github.com/whitequark/parser/#usage). Note that there is no support (yet) in main RuboCop gem. ([@marcandre][])
+* [#48](https://github.com/rubocop-hq/rubocop-ast/pull/48): Support `Parser::Ruby28` for Ruby 2.8 (3.0) parser. ([@koic][])
 
 ## 0.0.3 (2020-05-15)
 

lib/rubocop/ast/processed_source.rb

@@ -2,6 +2,7 @@
 
 require 'digest/sha1'
 
+# rubocop:disable Metrics/ClassLength
 module RuboCop
   module AST
     # ProcessedSource contains objects which are generated by Parser
@@ -176,6 +177,9 @@ def parser_class(ruby_version)
         when 2.7
           require 'parser/ruby27'
           Parser::Ruby27
+        when 2.8
+          require 'parser/ruby28'
+          Parser::Ruby28
         else
           raise ArgumentError,
                 "RuboCop found unknown Ruby version: #{ruby_version.inspect}"
@@ -201,3 +205,4 @@ def create_parser(ruby_version)
     end
   end
 end
+# rubocop:enable Metrics/ClassLength