Support Prism as a Ruby parser

This PR introduces the `parser_engine` option to `ProcessedSource` to support Prism,
as part of the RuboCop AST side effort towards addressing rubocop/rubocop#12600.

## Configuration

By default, analysis is performed using the Parser gem, so the default value
for the newly added `parser_engine` is `parser_whitequark`:

```ruby
ProcessedSource.new(@options[:stdin], ruby_version, file, parser_engine: :parser_whitequark)
```

This code maintains compatibility, meaning the traditional behavior is preserved:

```ruby
ProcessedSource.new(@options[:stdin], ruby_version, file)
```

To perform analysis using Prism, specify `parser_engine: :parser_prism`:

```ruby
ProcessedSource.new(@options[:stdin], ruby_version, file, parser_engine: :parser_prism)
```

The parameter name `parser_prism` reflects the original parser_prism which was the basis for
`Prism::Translation::Parser` (now integrated into Prism):
https://github.com/kddnewton/parser-prism

This is an experimental introduction, and some incompatibilities still remain.

> [!NOTE]
> As initially mentioned in rubocop/rubocop#12600 (comment),
> the plan was to set `parser_engine: prism`.
>
> However, the parser engine used in this PR is `Prism::Translation::Parser`, not `Prism`:
> ruby/prism#2419
>
> `Prism::Translation::Parser` and `Prism` have different ASTs, so their migration will definitely cause incompatibility.
> So, considering the possibility of further replacing `Prism::Translation::Parser` with `Prism` in the future,
> it has been decided that it might be better not to use `ParserEngine: prism` for the time being.
> `ParserEngine: prism` is reserved for `Prism`, not `Prism::Translation::Parser`.
>
> Therefore, the parameter value has been set to `parser_engine: parser_prism` specifically for
> `Prism::Translation::Parser`.
>
> This means that the planned way to specify Prism in .rubocop.yml file will be `ParserEngine: parser_prism`,
> not `ParserEngine: prism`.

## Compatibility

The compatibility issues between Prism and the Parser gem have not been resolved.
The failing tests will be skipped with `broken_on: :prism`:

- ruby/prism#2454 has been resolved but not yet released.
- ruby/prism#2467 is still unresolved.

Issues that will be resolved in several upcoming releases of Prism are being skipped
with `broken_on: :prism`.

Anyway, RuboCop AST can be released independently of the resolution and release of Prism.

> [!NOTE]
> The hack in `Prism::Translation::Parser` for `ProcessedSource` needs to be fixed:
> https://github.com/ruby/prism/blob/v0.24.0/lib/prism/translation/parser/rubocop.rb
>
> If the above interface is accepted, a fix will be proposed on the Prism side.

## Test

Tests for RuboCop AST with Prism as the backend can be run as follows:

```console
bundle exec rake prism_spec
```

The above is the shortcut alias for:

```console
PARSER_ENGINE=parser_prism TARGET_RUBY_VERSION=3.3 rake spec
```

RuboCop AST works on Ruby versions 2.6+, but since Prism only targets analysis for Ruby 3.3+,
`internal_investigation` Rake task will not be executed. This task is only run with the Parser gem,
which can analyze Ruby versions 2.0+.

.github/workflows/rubocop.yml

@@ -76,6 +76,23 @@ jobs:
       - name: spec
         if: "matrix.coverage != true && matrix.internal_investigation != true"
         run: bundle exec rake spec
+  prism:
+    runs-on: ubuntu-latest
+    name: Prism
+    steps:
+      - uses: actions/checkout@v4
+      - name: set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          # Specify the minimum Ruby version 2.7 required for Prism to run.
+          ruby-version: 2.7
+          bundler-cache: true
+      - name: spec
+        env:
+          # Specify the minimum Ruby version 3.3 required for Prism to analyze.
+          PARSER_ENGINE: parser_prism
+          TARGET_RUBY_VERSION: 3.3
+        run: bundle exec rake prism_spec
   rubocop_specs:
     name: >-
       Main Gem Specs | RuboCop: ${{ matrix.rubocop }} | ${{ matrix.ruby }}

Rakefile

@@ -22,6 +22,23 @@ RSpec::Core::RakeTask.new(spec: :generate) do |spec|
   spec.pattern = FileList['spec/**/*_spec.rb']
 end
 
+desc 'Run RSpec code examples with Prism'
+task prism_spec: :generate do
+  original_parser_engine = ENV.fetch('PARSER_ENGINE', nil)
+  original_target_ruby_version = ENV.fetch('TARGET_RUBY_VERSION', nil)
+
+  RSpec::Core::RakeTask.new(prism_spec: :generate) do |spec|
+    # Specify the minimum Ruby version 3.3 required for Prism to analyze.
+    ENV['PARSER_ENGINE'] = 'parser_prism'
+    ENV['TARGET_RUBY_VERSION'] = '3.3'
+
+    spec.pattern = FileList['spec/**/*_spec.rb']
+  end
+
+  ENV['PARSER_ENGINE'] = original_parser_engine
+  ENV['TARGET_RUBY_VERSION'] = original_target_ruby_version
+end
+
 desc 'Run RSpec with code coverage'
 task :coverage do
   ENV['COVERAGE'] = 'true'
@@ -35,5 +52,6 @@ end
 
 task default: %i[
   spec
+  prism_spec
   internal_investigation
 ]

changelog/new_support_prism.md

@@ -0,0 +1 @@
+* [#277](https://github.com/rubocop/rubocop-ast/pull/277): Support Prism as a Ruby parser (experimental). ([@koic][])

docs/modules/ROOT/pages/index.adoc

@@ -82,3 +82,14 @@ source = RuboCop::AST::ProcessedSource.new(code, 2.7)
 rule = MyRule.new
 source.ast.each_node { |n| rule.process(n) }
 ----
+
+In RuboCop AST, you can specify Prism as the parser engine backend by setting `parser_engine: :parser_prism`:
+
+```ruby
+# Using the Parser gem with `parser_engine: parser_whitequark` is the default.
+ProcessedSource.new(@options[:stdin], ruby_version, file, parser_engine: :parser_prism)
+```
+
+This is an experimental feature. If you encounter any incompatibilities between
+Prism and the Parser gem, please check the following URL:
+https://github.com/ruby/prism/issues?q=is%3Aissue+is%3Aopen+label%3Arubocop

lib/rubocop/ast/processed_source.rb

@@ -15,15 +15,23 @@ class ProcessedSource
       INVALID_LEVELS = %i[error fatal].freeze
       private_constant :INVALID_LEVELS
 
+      PARSER_ENGINES = %i[parser_whitequark parser_prism].freeze
+      private_constant :PARSER_ENGINES
+
       attr_reader :path, :buffer, :ast, :comments, :tokens, :diagnostics,
-                  :parser_error, :raw_source, :ruby_version
+                  :parser_error, :raw_source, :ruby_version, :parser_engine
 
-      def self.from_file(path, ruby_version)
+      def self.from_file(path, ruby_version, parser_engine: :parser_whitequark)
         file = File.read(path, mode: 'rb')
-        new(file, ruby_version, path)
+        new(file, ruby_version, path, parser_engine: parser_engine)
       end
 
-      def initialize(source, ruby_version, path = nil)
+      def initialize(source, ruby_version, path = nil, parser_engine: :parser_whitequark)
+        unless PARSER_ENGINES.include?(parser_engine)
+          raise ArgumentError, 'The keyword argument `parser_engine` accepts ' \
+                               "`parser` or `parser_prism`, but `#{parser_engine}` was passed."
+        end
+
         # Defaults source encoding to UTF-8, regardless of the encoding it has
         # been read with, which could be non-utf8 depending on the default
         # external encoding.
@@ -33,9 +41,10 @@ def initialize(source, ruby_version, path = nil)
         @path = path
         @diagnostics = []
         @ruby_version = ruby_version
+        @parser_engine = parser_engine
         @parser_error = nil
 
-        parse(source, ruby_version)
+        parse(source, ruby_version, parser_engine)
       end
 
       def ast_with_comments
@@ -193,7 +202,7 @@ def comment_index
         end
       end
 
-      def parse(source, ruby_version)
+      def parse(source, ruby_version, parser_engine)
         buffer_name = @path || STRING_SOURCE_NAME
         @buffer = Parser::Source::Buffer.new(buffer_name, 1)
 
@@ -207,7 +216,7 @@ def parse(source, ruby_version)
           return
         end
 
-        @ast, @comments, @tokens = tokenize(create_parser(ruby_version))
+        @ast, @comments, @tokens = tokenize(create_parser(ruby_version, parser_engine))
       end
 
       def tokenize(parser)
@@ -227,61 +236,77 @@ def tokenize(parser)
       end
 
       # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength
-      def parser_class(ruby_version)
-        case ruby_version
-        when 1.9
-          require 'parser/ruby19'
-          Parser::Ruby19
-        when 2.0
-          require 'parser/ruby20'
-          Parser::Ruby20
-        when 2.1
-          require 'parser/ruby21'
-          Parser::Ruby21
-        when 2.2
-          require 'parser/ruby22'
-          Parser::Ruby22
-        when 2.3
-          require 'parser/ruby23'
-          Parser::Ruby23
-        when 2.4
-          require 'parser/ruby24'
-          Parser::Ruby24
-        when 2.5
-          require 'parser/ruby25'
-          Parser::Ruby25
-        when 2.6
-          require 'parser/ruby26'
-          Parser::Ruby26
-        when 2.7
-          require 'parser/ruby27'
-          Parser::Ruby27
-        when 2.8, 3.0
-          require 'parser/ruby30'
-          Parser::Ruby30
-        when 3.1
-          require 'parser/ruby31'
-          Parser::Ruby31
-        when 3.2
-          require 'parser/ruby32'
-          Parser::Ruby32
-        when 3.3
-          require 'parser/ruby33'
-          Parser::Ruby33
-        when 3.4
-          require 'parser/ruby34'
-          Parser::Ruby34
-        else
-          raise ArgumentError,
-                "RuboCop found unknown Ruby version: #{ruby_version.inspect}"
+      def parser_class(ruby_version, parser_engine)
+        case parser_engine
+        when :parser_whitequark
+          case ruby_version
+          when 1.9
+            require 'parser/ruby19'
+            Parser::Ruby19
+          when 2.0
+            require 'parser/ruby20'
+            Parser::Ruby20
+          when 2.1
+            require 'parser/ruby21'
+            Parser::Ruby21
+          when 2.2
+            require 'parser/ruby22'
+            Parser::Ruby22
+          when 2.3
+            require 'parser/ruby23'
+            Parser::Ruby23
+          when 2.4
+            require 'parser/ruby24'
+            Parser::Ruby24
+          when 2.5
+            require 'parser/ruby25'
+            Parser::Ruby25
+          when 2.6
+            require 'parser/ruby26'
+            Parser::Ruby26
+          when 2.7
+            require 'parser/ruby27'
+            Parser::Ruby27
+          when 2.8, 3.0
+            require 'parser/ruby30'
+            Parser::Ruby30
+          when 3.1
+            require 'parser/ruby31'
+            Parser::Ruby31
+          when 3.2
+            require 'parser/ruby32'
+            Parser::Ruby32
+          when 3.3
+            require 'parser/ruby33'
+            Parser::Ruby33
+          when 3.4
+            require 'parser/ruby34'
+            Parser::Ruby34
+          else
+            raise ArgumentError, "RuboCop found unknown Ruby version: #{ruby_version.inspect}"
+          end
+        when :parser_prism
+          require 'prism'
+
+          case ruby_version
+          when 3.3
+            require 'prism/translation/parser33'
+            Prism::Translation::Parser33
+          when 3.4
+            require 'prism/translation/parser34'
+            Prism::Translation::Parser34
+          else
+            raise ArgumentError, 'RuboCop supports target Ruby versions 3.3 and above with Prism. ' \
+                                 "Specified target Ruby version: #{ruby_version.inspect}"
+          end
         end
       end
       # rubocop:enable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength
 
-      def create_parser(ruby_version)
+      def create_parser(ruby_version, parser_engine)
         builder = RuboCop::AST::Builder.new
 
-        parser_class(ruby_version).new(builder).tap do |parser|
+        parser_class(ruby_version, parser_engine).new(builder).tap do |parser|
           # On JRuby there's a risk that we hang in tokenize() if we
           # don't set the all errors as fatal flag. The problem is caused by a bug
           # in Racc that is discussed in issue #93 of the whitequark/parser

rubocop-ast.gemspec

@@ -34,6 +34,7 @@ Gem::Specification.new do |s|
   }
 
   s.add_runtime_dependency('parser', '>= 3.3.0.4')
+  s.add_runtime_dependency('prism', '>= 0.24.0')
 
   ##### Do NOT add `rubocop` (or anything depending on `rubocop`) here. See Gemfile
 end

spec/rubocop/ast/if_node_spec.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
-RSpec.describe RuboCop::AST::IfNode do
+# FIXME: `broken_on: :prism` can be removed when Prism > 0.24.0 will be released.
+RSpec.describe RuboCop::AST::IfNode, broken_on: :prism do
   subject(:if_node) { parse_source(source).ast }
 
   describe '.new' do

spec/rubocop/ast/processed_source_spec.rb

@@ -1,7 +1,9 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::AST::ProcessedSource do
-  subject(:processed_source) { described_class.new(source, ruby_version, path) }
+  subject(:processed_source) do
+    described_class.new(source, ruby_version, path, parser_engine: parser_engine)
+  end
 
   let(:source) { <<~RUBY }
     # an awesome method
@@ -25,6 +27,17 @@ def some_method
         is_expected.to be_a(described_class)
       end
     end
+
+    context 'when using invalid `parser_engine` argument' do
+      let(:parser_engine) { :unknown_parser_engine }
+
+      it 'raises a Errno::ENOENT when the file does not exist' do
+        expect { processed_source }.to raise_error(ArgumentError) do |e|
+          expect(e.message).to eq 'The keyword argument `parser_engine` accepts `parser` or ' \
+                                  '`parser_prism`, but `unknown_parser_engine` was passed.'
+        end
+      end
+    end
   end
 
   describe '.from_file' do
@@ -36,7 +49,9 @@ def some_method
         Dir.chdir(org_pwd)
       end
 
-      let(:processed_source) { described_class.from_file(path, ruby_version) }
+      let(:processed_source) do
+        described_class.from_file(path, ruby_version, parser_engine: parser_engine)
+      end
 
       it 'returns an instance of ProcessedSource' do
         is_expected.to be_a(described_class)
@@ -186,7 +201,9 @@ def some_method
       end
     end
 
-    context 'when the source is valid but has some warning diagnostics' do
+    # FIXME: `broken_on: :prism` can be removed when
+    # https://github.com/ruby/prism/issues/2454 will be released.
+    context 'when the source is valid but has some warning diagnostics', broken_on: :prism do
       let(:source) { 'do_something *array' }
 
       it 'returns true' do
@@ -442,7 +459,8 @@ def some_method
   end
   # rubocop:enable RSpec/RedundantPredicateMatcher
 
-  describe '#preceding_line' do
+  # FIXME: https://github.com/ruby/prism/issues/2467
+  describe '#preceding_line', broken_on: :prism do
     let(:source) { <<~RUBY }
       [ line, 1 ]
       { line: 2 }
@@ -458,7 +476,8 @@ def some_method
     end
   end
 
-  describe '#following_line' do
+  # FIXME: https://github.com/ruby/prism/issues/2467
+  describe '#following_line', broken_on: :prism do
     let(:source) { <<~RUBY }
       [ line, 1 ]
       { line: 2 }

spec/rubocop/ast/range_node_spec.rb

@@ -22,8 +22,7 @@
       it { is_expected.to be_range_type }
     end
 
-    context 'with an infinite range' do
-      let(:ruby_version) { 2.6 }
+    context 'with an infinite range', :ruby26 do
       let(:source) do
         '1..'
       end
@@ -32,8 +31,7 @@
       it { is_expected.to be_range_type }
     end
 
-    context 'with a beignless range' do
-      let(:ruby_version) { 2.7 }
+    context 'with a beignless range', :ruby27 do
       let(:source) do
         '..42'
       end

spec/rubocop/ast/token_spec.rb

@@ -342,7 +342,9 @@ def foo
       end
 
       describe '#left_brace?' do
-        it 'returns true for left hash brace tokens' do
+        # FIXME: `broken_on: :prism` can be removed when
+        # https://github.com/ruby/prism/issues/2454 will be released.
+        it 'returns true for left hash brace tokens', broken_on: :prism do
           expect(left_hash_brace_token).to be_left_brace
         end
 
@@ -357,7 +359,9 @@ def foo
           expect(left_block_brace_token).to be_left_curly_brace
         end
 
-        it 'returns false for non left block brace tokens' do
+        # FIXME: `broken_on: :prism` can be removed when
+        # https://github.com/ruby/prism/issues/2454 will be released.
+        it 'returns false for non left block brace tokens', broken_on: :prism do
           expect(left_hash_brace_token).not_to be_left_curly_brace
           expect(right_block_brace_token).not_to be_left_curly_brace
         end

spec/rubocop/ast/traversal_spec.rb

@@ -86,7 +86,8 @@ def initialize
 
       let(:source) { "foo=bar=baz=nil; #{example}" }
 
-      it 'traverses all nodes' do
+      # FIXME: `broken_on: :prism` can be removed when Prism > 0.24.0 will be released.
+      it 'traverses all nodes', broken_on: :prism do
         actual = node.each_node.count
         expect(traverse.hits).to eql(actual)
       end