Add AST::InPatternNode node

This PR adds `AST::InPatternNode` node.
It aims to make access to `in` pattern node of the pattern matching syntax
convenient and readable.

For example, pattern matching syntax AST can use `in_pattern.body` and
prevents the following error:

```console
NoMethodError:
  undefined method `body' for s(:in_pattern,
    s(:array_pattern,
      s(:match_var, :a)), nil,
    s(:int, 1)):RuboCop::AST::Node
```

changelog/new_add_ast_in_pattern_node.md

@@ -0,0 +1 @@
+* [#183](https://github.com/rubocop-hq/rubocop-ast/pull/183): Add `AST::InPatternNode` node. ([@koic][])

lib/rubocop/ast.rb

@@ -53,6 +53,7 @@
 require_relative 'ast/node/float_node'
 require_relative 'ast/node/hash_node'
 require_relative 'ast/node/if_node'
+require_relative 'ast/node/in_pattern_node'
 require_relative 'ast/node/index_node'
 require_relative 'ast/node/indexasgn_node'
 require_relative 'ast/node/int_node'

lib/rubocop/ast/builder.rb

@@ -49,6 +49,7 @@ class Builder < Parser::Builders::Default
         float:        FloatNode,
         hash:         HashNode,
         if:           IfNode,
+        in_pattern:   InPatternNode,
         int:          IntNode,
         index:        IndexNode,
         indexasgn:    IndexasgnNode,

lib/rubocop/ast/node/case_match_node.rb

@@ -24,9 +24,9 @@ def each_in_pattern(&block)
         self
       end
 
-      # Returns an array of all the when branches in the `case` statement.
+      # Returns an array of all the `in` pattern branches in the `case` statement.
       #
-      # @return [Array<Node>] an array of `in_pattern` nodes
+      # @return [Array<InPatternNode>] an array of `in_pattern` nodes
       def in_pattern_branches
         node_parts[1...-1]
       end

lib/rubocop/ast/node/in_pattern_node.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module AST
+    # A node extension for `in` nodes. This will be used in place of a plain
+    # node when the builder constructs the AST, making its methods available
+    # to all `in` nodes within RuboCop.
+    class InPatternNode < Node
+      # Returns the index of the `in` branch within the `case` statement.
+      #
+      # @return [Integer] the index of the `in` branch
+      def branch_index
+        parent.in_pattern_branches.index(self)
+      end
+
+      # Checks whether the `in` node has a `then` keyword.
+      #
+      # @return [Boolean] whether the `in` node has a `then` keyword
+      def then?
+        loc.begin&.is?('then')
+      end
+
+      # Returns the body of the `in` node.
+      #
+      # @return [Node, nil] the body of the `in` node
+      def body
+        node_parts[-1]
+      end
+    end
+  end
+end

spec/rubocop/ast/in_pattern_node_spec.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::AST::InPatternNode do
+  context 'when using Ruby 2.7 or newer', :ruby27 do
+    let(:in_pattern_node) { parse_source(source).ast.children[1] }
+
+    describe '.new' do
+      let(:source) do
+        ['case condition',
+         'in [42] then foo',
+         'end'].join("\n")
+      end
+
+      it { expect(in_pattern_node).to be_a(described_class) }
+    end
+
+    describe '#then?' do
+      context 'with a then keyword' do
+        let(:source) do
+          ['case condition',
+           'in [42] then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node).to be_then }
+      end
+
+      context 'without a then keyword' do
+        let(:source) do
+          ['case condition',
+           'in [42]',
+           '  foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node).not_to be_then }
+      end
+    end
+
+    describe '#body' do
+      context 'with a then keyword' do
+        let(:source) do
+          ['case condition',
+           'in [42] then :foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.body).to be_sym_type }
+      end
+
+      context 'without a then keyword' do
+        let(:source) do
+          ['case condition',
+           'in [42]',
+           '  [:foo, :bar]',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.body).to be_array_type }
+      end
+    end
+
+    describe '#branch_index' do
+      let(:source) do
+        ['case condition',
+         'in [42] then 1',
+         'in [43] then 2',
+         'in [44] then 3',
+         'end'].join("\n")
+      end
+
+      let(:in_patterns) { parse_source(source).ast.children[1...-1] }
+
+      it { expect(in_patterns[0].branch_index).to eq(0) }
+      it { expect(in_patterns[1].branch_index).to eq(1) }
+      it { expect(in_patterns[2].branch_index).to eq(2) }
+    end
+  end
+end