Enable Lint/UselessAssignment cop to avoid unused variable warnings (#34904)

* Enable `Lint/UselessAssignment` cop to avoid unused variable warnings

Since we've addressed the warning "assigned but unused variable"
frequently.

370537de05
3040446cec
5ed618e192
76ebafe594

And also, I've found the unused args in c1b14ad which raises no warnings
by the cop, it shows the value of the cop.

.rubocop.yml

@@ -191,6 +191,9 @@ Lint/StringConversionInInterpolation:
 Lint/UriEscapeUnescape:
   Enabled: true
 
+Lint/UselessAssignment:
+  Enabled: true
+
 Lint/DeprecatedClassMethods:
   Enabled: true
 

actionpack/test/controller/filters_test.rb

@@ -888,7 +888,7 @@ class ControllerWithSymbolAsFilter < PostsController
       yield
 
       # Do stuff...
-      wtf += 1
+      wtf + 1
     end
 end
 

actionpack/test/controller/webservice_test.rb

@@ -14,7 +14,7 @@ class WebServiceTest < ActionDispatch::IntegrationTest
     end
 
     def dump_params_keys(hash = params)
-      hash.keys.sort.inject("") do |s, k|
+      hash.keys.sort.each_with_object(+"") do |k, s|
         value = hash[k]
 
         if value.is_a?(Hash) || value.is_a?(ActionController::Parameters)
@@ -23,8 +23,8 @@ class WebServiceTest < ActionDispatch::IntegrationTest
           value = ""
         end
 
-        s += ", " unless s.empty?
-        s += "#{k}#{value}"
+        s << ", " unless s.empty?
+        s << "#{k}#{value}"
       end
     end
   end

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -22,8 +22,8 @@ module ActiveRecord
         def create_database(name, options = {})
           options = { encoding: "utf8" }.merge!(options.symbolize_keys)
 
-          option_string = options.inject("") do |memo, (key, value)|
-            memo += case key
+          option_string = options.each_with_object(+"") do |(key, value), memo|
+            memo << case key
                     when :owner
                       " OWNER = \"#{value}\""
                     when :template

activerecord/lib/active_record/timestamp.rb

@@ -56,7 +56,7 @@ module ActiveRecord
       def touch_attributes_with_time(*names, time: nil)
         attribute_names = timestamp_attributes_for_update_in_model
         attribute_names |= names.map(&:to_s)
-        attribute_names.index_with(time ||= current_time_from_proper_timezone)
+        attribute_names.index_with(time || current_time_from_proper_timezone)
       end
 
       private

activerecord/lib/arel/visitors/informix.rb

@@ -15,8 +15,9 @@ module Arel # :nodoc: all
             collector << "ORDER BY "
             collector = inject_join o.orders, collector, ", "
           end
-          collector = maybe_visit o.lock, collector
+          maybe_visit o.lock, collector
         end
+
         def visit_Arel_Nodes_SelectCore(o, collector)
           collector = inject_join o.projections, collector, ", "
           if o.source && !o.source.empty?

activerecord/lib/arel/visitors/oracle12.rb

@@ -20,7 +20,7 @@ module Arel # :nodoc: all
         def visit_Arel_Nodes_SelectOptions(o, collector)
           collector = maybe_visit o.offset, collector
           collector = maybe_visit o.limit, collector
-          collector = maybe_visit o.lock, collector
+          maybe_visit o.lock, collector
         end
 
         def visit_Arel_Nodes_Limit(o, collector)

activerecord/lib/arel/visitors/to_sql.rb

@@ -208,14 +208,12 @@ module Arel # :nodoc: all
           end
 
           visit_Arel_Nodes_SelectOptions(o, collector)
-
-          collector
         end
 
         def visit_Arel_Nodes_SelectOptions(o, collector)
           collector = maybe_visit o.limit, collector
           collector = maybe_visit o.offset, collector
-          collector = maybe_visit o.lock, collector
+          maybe_visit o.lock, collector
         end
 
         def visit_Arel_Nodes_SelectCore(o, collector)

activerecord/test/cases/batches_test.rb

@@ -430,7 +430,7 @@ class EachTest < ActiveRecord::TestCase
         assert_kind_of ActiveRecord::Relation, relation
         assert_kind_of Post, relation.first
 
-        relation = [not_a_post] * relation.count
+        [not_a_post] * relation.count
       end
     end
   end

activerecord/test/cases/counter_cache_test.rb

@@ -144,7 +144,7 @@ class CounterCacheTest < ActiveRecord::TestCase
 
   test "update other counters on parent destroy" do
     david, joanna = dog_lovers(:david, :joanna)
-    joanna = joanna # squelch a warning
+    _ = joanna # squelch a warning
 
     assert_difference "joanna.reload.dogs_count", -1 do
       david.destroy

activerecord/test/cases/inheritance_test.rb

@@ -240,7 +240,7 @@ class InheritanceTest < ActiveRecord::TestCase
     cabbage = vegetable.becomes!(Cabbage)
     assert_equal "Cabbage", cabbage.custom_type
 
-    vegetable = cabbage.becomes!(Vegetable)
+    cabbage.becomes!(Vegetable)
     assert_nil cabbage.custom_type
   end
 
@@ -654,7 +654,7 @@ class InheritanceAttributeMappingTest < ActiveRecord::TestCase
 
     assert_equal ["omg_inheritance_attribute_mapping_test/company"], ActiveRecord::Base.connection.select_values("SELECT sponsorable_type FROM sponsors")
 
-    sponsor = Sponsor.first
+    sponsor = Sponsor.find(sponsor.id)
     assert_equal startup, sponsor.sponsorable
   end
 end

activesupport/test/test_case_test.rb

@@ -104,7 +104,7 @@ class AssertionsTest < ActiveSupport::TestCase
   def test_expression_is_evaluated_in_the_appropriate_scope
     silence_warnings do
       local_scope = "foo"
-      local_scope = local_scope  # to suppress unused variable warning
+      _ = local_scope  # to suppress unused variable warning
       assert_difference("local_scope; @object.num") { @object.increment }
     end
   end

guides/rails_guides/generator.rb

@@ -63,9 +63,9 @@ module RailsGuides
       end
 
       def mobi
-        mobi  = "ruby_on_rails_guides_#{@version || @edge[0, 7]}"
-        mobi += ".#{@language}" if @language
-        mobi += ".mobi"
+        mobi = +"ruby_on_rails_guides_#{@version || @edge[0, 7]}"
+        mobi << ".#{@language}" if @language
+        mobi << ".mobi"
       end
 
       def initialize_dirs

railties/lib/rails/code_statistics.rb

@@ -95,8 +95,8 @@ class CodeStatistics #:nodoc:
     end
 
     def print_line(name, statistics)
-      m_over_c   = (statistics.methods / statistics.classes) rescue m_over_c = 0
-      loc_over_m = (statistics.code_lines / statistics.methods) - 2 rescue loc_over_m = 0
+      m_over_c   = (statistics.methods / statistics.classes) rescue 0
+      loc_over_m = (statistics.code_lines / statistics.methods) - 2 rescue 0
 
       print "| #{name.ljust(20)} "
       HEADERS.each_key do |k|

railties/lib/rails/generators/rails/plugin/plugin_generator.rb

@@ -349,9 +349,9 @@ task default: :test
       def wrap_in_modules(unwrapped_code)
         unwrapped_code = "#{unwrapped_code}".strip.gsub(/\s$\n/, "")
         modules.reverse.inject(unwrapped_code) do |content, mod|
-          str = "module #{mod}\n"
-          str += content.lines.map { |line| "  #{line}" }.join
-          str += content.present? ? "\nend" : "end"
+          str = +"module #{mod}\n"
+          str << content.lines.map { |line| "  #{line}" }.join
+          str << (content.present? ? "\nend" : "end")
         end
       end