Performance/Sum should avoid empty arrays

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changes
 
 * [#170](https://github.com/rubocop-hq/rubocop-performance/pull/170): Extend `Performance/Sum` to register an offense for `map { ... }.sum`. ([@eugeneius][])
+* [#179](https://github.com/rubocop-hq/rubocop-performance/pull/179): Change `Performance/Sum` to not error on empty array literals. ([@ghiculescu][])
 
 ## 1.8.1 (2020-09-19)
 
@@ -174,3 +175,4 @@
 [@siegfault]: https://github.com/siegfault
 [@fatkodima]: https://github.com/fatkodima
 [@dvandersluis]: https://github.com/dvandersluis
+[@ghiculescu]: https://github.com/ghiculescu

lib/rubocop/cop/performance/sum.rb

@@ -53,6 +53,28 @@ class Sum < Base
         alias elem_plus_acc? acc_plus_elem?
 
         def on_send(node)
+          return if empty_array_literal?(node.children.first)
+
+          handle_sum_candidate(node)
+          handle_sum_map_candidate(node)
+        end
+
+        def on_block(node)
+          sum_with_block_candidate?(node) do |send, init, var_acc, var_elem, body|
+            if acc_plus_elem?(body, var_acc, var_elem) || elem_plus_acc?(body, var_elem, var_acc)
+              range = sum_block_range(send, node)
+              message = build_block_message(send, init, var_acc, var_elem, body)
+
+              add_offense(range, message: message) do |corrector|
+                autocorrect(corrector, init, range)
+              end
+            end
+          end
+        end
+
+        private
+
+        def handle_sum_candidate(node)
           sum_candidate?(node) do |method, init, operation|
             range = sum_method_range(node)
             message = build_method_message(method, init, operation)
@@ -61,7 +83,9 @@ def on_send(node)
               autocorrect(corrector, init, range)
             end
           end
+        end
 
+        def handle_sum_map_candidate(node)
           sum_map_candidate?(node) do |map, init|
             next if node.block_literal? || node.block_argument?
 
@@ -73,21 +97,10 @@ def on_send(node)
           end
         end
 
-        def on_block(node)
-          sum_with_block_candidate?(node) do |send, init, var_acc, var_elem, body|
-            if acc_plus_elem?(body, var_acc, var_elem) || elem_plus_acc?(body, var_elem, var_acc)
-              range = sum_block_range(send, node)
-              message = build_block_message(send, init, var_acc, var_elem, body)
-
-              add_offense(range, message: message) do |corrector|
-                autocorrect(corrector, init, range)
-              end
-            end
-          end
+        def empty_array_literal?(node)
+          !node.nil? && node.literal? && node.array_type? && node.children.empty?
         end
 
-        private
-
         def autocorrect(corrector, init, range)
           return if init.empty?
 

spec/rubocop/cop/performance/sum_spec.rb

@@ -99,6 +99,22 @@
       expect_no_corrections
     end
 
+    # ideally it would autocorrect to `[1, 2, 3].sum`
+    it 'registers an offense but does not autocorrect on array literals' do
+      expect_offense(<<~RUBY, method: method)
+        [1, 2, 3].#{method}(:+)
+                  ^{method}^^^^ Use `sum` instead of `#{method}(:+)`.
+      RUBY
+
+      expect_no_corrections
+    end
+
+    it 'does not register an offense when the array is empty' do
+      expect_no_offenses(<<~RUBY, method: method)
+        [].#{method}(:+)
+      RUBY
+    end
+
     it 'does not register an offense when block does not implement summation' do
       expect_no_offenses(<<~RUBY)
         array.#{method} { |acc, elem| elem * 2 }