Add RuboCop::AST::RationalNode

This PR resolves the fundamental issue mentioned in rubocop/rubocop#13051.
Although rational literal (e.g., `(0.2r)` is a basic literal that return `true` for `RuboCop::AST::Node#numeric_type?`
and `RuboCop::AST::Node#literal?`, their behavior was not sufficiently covered as a `NumericNode`.

This PR maps rational literal to `RationalNode` to enable expected node operations for rational literals.

changelog/new_add_rational_node.md

@@ -0,0 +1 @@
+* [#304](https://github.com/rubocop/rubocop-ast/pull/304): Add `RuboCop::AST::RationalNode`. ([@koic][])

lib/rubocop/ast.rb

@@ -69,6 +69,7 @@
 require_relative 'ast/node/pair_node'
 require_relative 'ast/node/procarg0_node'
 require_relative 'ast/node/range_node'
+require_relative 'ast/node/rational_node'
 require_relative 'ast/node/regexp_node'
 require_relative 'ast/node/rescue_node'
 require_relative 'ast/node/resbody_node'

lib/rubocop/ast/builder.rb

@@ -72,6 +72,7 @@ class Builder < Parser::Builders::Default
         or:                  OrNode,
         pair:                PairNode,
         procarg0:            Procarg0Node,
+        rational:            RationalNode,
         regexp:              RegexpNode,
         rescue:              RescueNode,
         resbody:             ResbodyNode,

lib/rubocop/ast/node/mixin/basic_literal_node.rb

@@ -3,7 +3,7 @@
 module RuboCop
   module AST
     # Common functionality for primitive literal nodes: `sym`, `str`,
-    # `int`, `float`, ...
+    # `int`, `float`, `rational`...
     module BasicLiteralNode
       # Returns the value of the literal.
       #

lib/rubocop/ast/node/mixin/numeric_node.rb

@@ -2,7 +2,7 @@
 
 module RuboCop
   module AST
-    # Common functionality for primitive numeric nodes: `int`, `float`, ...
+    # Common functionality for primitive numeric nodes: `int`, `float`, `rational`...
     module NumericNode
       SIGN_REGEX = /\A[+-]/.freeze
       private_constant :SIGN_REGEX

lib/rubocop/ast/node/rational_node.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module AST
+    # A node extension for `rational` nodes. This will be used in place of a plain
+    # node when the builder constructs the AST, making its methods available to
+    # all `rational` nodes within RuboCop.
+    class RationalNode < Node
+      include BasicLiteralNode
+      include NumericNode
+    end
+  end
+end

spec/rubocop/ast/rational_node_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::AST::RationalNode do
+  subject(:rational_node) { parse_source(source).ast }
+
+  describe '.new' do
+    let(:source) { '0.2r' }
+
+    it { is_expected.to be_a(described_class) }
+  end
+
+  describe '#sign?' do
+    context 'when explicit positive rational' do
+      let(:source) { '+0.2r' }
+
+      it { is_expected.to be_sign }
+    end
+
+    context 'when explicit negative rational' do
+      let(:source) { '-0.2r' }
+
+      it { is_expected.to be_sign }
+    end
+  end
+
+  describe '#value' do
+    let(:source) do
+      '0.4r'
+    end
+
+    it { expect(rational_node.value).to eq(0.4r) }
+  end
+end