Update Lint/UnmodifiedReduceAccumulator to accept blocks which retu…

…rn in the form `accumulator[element]`.

changelog/change_update_lint_unmodified_reduce_accumulator.md

@@ -0,0 +1 @@
+* [#9059](https://github.com/rubocop-hq/rubocop/pull/9059): Update `Lint/UnmodifiedReduceAccumulator` to accept blocks which return in the form `accumulator[element]`. ([@dvandersluis][])

lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb

@@ -54,6 +54,9 @@ module Lint
       #     value
       #   end
       #
+      #   # good, recursive
+      #   keys.reduce(self) { |result, key| result[key] }
+      #
       #   # ignored as the return value cannot be determined
       #   enum.reduce do |acc, el|
       #     x = foo(acc, el)
@@ -131,7 +134,7 @@ def check_return_values(block_node)
           element_name = block_arg_name(block_node, 1)
           message_opts = { method: block_node.method_name, accum: accumulator_name }
 
-          if (node = returned_accumulator_index(return_values, accumulator_name))
+          if (node = returned_accumulator_index(return_values, accumulator_name, element_name))
             add_offense(node, message: format(MSG_INDEX, message_opts))
           elsif potential_offense?(return_values, block_node.body, element_name, accumulator_name)
             return_values.each do |return_val|
@@ -146,11 +149,17 @@ def block_arg_name(node, index)
           node.arguments[index].node_parts[0]
         end
 
-        # Look for an index of the accumulator being returned
+        # Look for an index of the accumulator being returned, except where the index
+        # is the element.
         # This is always an offense, in order to try to catch potential exceptions
         # due to type mismatches
-        def returned_accumulator_index(return_values, accumulator_name)
-          return_values.detect { |val| accumulator_index?(val, accumulator_name) }
+        def returned_accumulator_index(return_values, accumulator_name, element_name)
+          return_values.detect do |val|
+            next unless accumulator_index?(val, accumulator_name)
+            next true if val.method?(:[]=)
+
+            val.arguments.none? { |arg| lvar_used?(arg, element_name) }
+          end
         end
 
         def potential_offense?(return_values, block_body, element_name, accumulator_name)

spec/rubocop/cop/lint/unmodified_reduce_accumulator_spec.rb

@@ -373,15 +373,43 @@
         RUBY
       end
 
+      it 'does not register an offense when calling a method on the accumulator with the element' do
+        expect_no_offenses(<<~RUBY)
+          foo.#{method} { |result, key| result.method(key) }
+        RUBY
+      end
+
+      it 'registers an offense when returning an index of the accumulator' do
+        expect_offense(<<~RUBY)
+          %w(a b c).#{method}({}) do |acc, letter|
+            acc[foo]
+            ^^^^^^^^ Do not return an element of the accumulator in `#{method}`.
+          end
+        RUBY
+      end
+
       it 'registers an offense when returning an index setter on the accumulator' do
         expect_offense(<<~RUBY)
           %w(a b c).#{method}({}) do |acc, letter|
-            acc[letter] = true
-            ^^^^^^^^^^^^^^^^^^ Do not return an element of the accumulator in `#{method}`.
+            acc[foo] = bar
+            ^^^^^^^^^^^^^^ Do not return an element of the accumulator in `#{method}`.
           end
         RUBY
       end
 
+      it 'does not register an offense when returning accumulator[element]' do
+        expect_no_offenses(<<~RUBY)
+          foo.#{method} { |result, key| result[key] }
+        RUBY
+      end
+
+      it 'registers an offense when returning accumulator[element]=' do
+        expect_offense(<<~RUBY, method: method)
+          foo.#{method} { |result, key| result[key] = foo }
+              _{method}                 ^^^^^^^^^^^^^^^^^ Do not return an element of the accumulator in `#{method}`.
+        RUBY
+      end
+
       it 'registers an offense when returning an expression with the element' do
         expect_offense(<<~RUBY)
           (1..4).#{method}(0) do |acc, el|