Add pattern method for AST::InPatternNode

This PR adds `pattern` method for `AST::InPatternNode`.

changelog/new_add_pattern_method_for_ast_in_pattern_node

@@ -0,0 +1 @@
+* [#186](https://github.com/rubocop-hq/rubocop-ast/pull/186): Add `pattern` method for `AST::InPatternNode` node. ([@koic][])

lib/rubocop/ast/node/in_pattern_node.rb

@@ -6,6 +6,13 @@ module AST
     # node when the builder constructs the AST, making its methods available
     # to all `in` nodes within RuboCop.
     class InPatternNode < Node
+      # Returns a node of the pattern in the `in` branch.
+      #
+      # @return [Node] a pattern node
+      def pattern
+        node_parts.first
+      end
+
       # Returns the index of the `in` branch within the `case` statement.
       #
       # @return [Integer] the index of the `in` branch

spec/rubocop/ast/in_pattern_node_spec.rb

@@ -14,6 +14,78 @@
       it { expect(in_pattern_node).to be_a(described_class) }
     end
 
+    describe '#pattern' do
+      context 'with a value pattern' do
+        let(:source) do
+          ['case condition',
+           'in 42 then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_int_type }
+      end
+
+      context 'with a variable pattern' do
+        let(:source) do
+          ['case condition',
+           'in var then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_match_var_type }
+      end
+
+      context 'with an alternative pattern' do
+        let(:source) do
+          ['case condition',
+           'in :foo | :bar | :baz then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_match_alt_type }
+      end
+
+      context 'with an as pattern' do
+        let(:source) do
+          ['case condition',
+           'in Integer => var then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_match_as_type }
+      end
+
+      context 'with an array pattern' do
+        let(:source) do
+          ['case condition',
+           'in :foo, :bar, :baz then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_array_pattern_type }
+      end
+
+      context 'with a hash pattern' do
+        let(:source) do
+          ['case condition',
+           'in foo:, bar:, baz: then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_hash_pattern_type }
+      end
+
+      context 'with a pin operator', :ruby31 do
+        let(:source) do
+          ['case condition',
+           'in ^(2 + 2) then foo',
+           'end'].join("\n")
+        end
+
+        it { expect(in_pattern_node.pattern).to be_pin_type }
+      end
+    end
+
     describe '#then?' do
       context 'with a then keyword' do
         let(:source) do