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.

370537de05092aeea552146b42042833212a1acc
3040446cece8e7a6d9e29219e636e13f180a1e03
5ed618e192e9788094bd92c51255dda1c4fd0eae
76ebafe594fc23abc3764acc7a3758ca473799e5

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

15 files changed, 27 insertions, 25 deletions

diff --git a/.rubocop.yml b/.rubocop.yml
index e5239063ac..4d2bacde32 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -191,6 +191,9 @@ Lint/StringConversionInInterpolation:
 Lint/UriEscapeUnescape:
   Enabled: true
 
+Lint/UselessAssignment:
+  Enabled: true
+
 Lint/DeprecatedClassMethods:
   Enabled: true
 
diff --git a/actionpack/test/controller/filters_test.rb b/actionpack/test/controller/filters_test.rb
index 104c9eeade..fcee812ee4 100644
--- a/actionpack/test/controller/filters_test.rb
+++ b/actionpack/test/controller/filters_test.rb
@@ -888,7 +888,7 @@ class ControllerWithSymbolAsFilter < PostsController
       yield
 
       # Do stuff...
-      wtf += 1
+      wtf + 1
     end
 end
 
diff --git a/actionpack/test/controller/webservice_test.rb b/actionpack/test/controller/webservice_test.rb
index 4a10637b54..23a46df5cd 100644
--- a/actionpack/test/controller/webservice_test.rb
+++ b/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
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 16260fe565..3516bef75a 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/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
diff --git a/activerecord/lib/active_record/timestamp.rb b/activerecord/lib/active_record/timestamp.rb
index d32f971ad1..e19077eb88 100644
--- a/activerecord/lib/active_record/timestamp.rb
+++ b/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
diff --git a/activerecord/lib/arel/visitors/informix.rb b/activerecord/lib/arel/visitors/informix.rb
index 0a9713794e..208fa15aef 100644
--- a/activerecord/lib/arel/visitors/informix.rb
+++ b/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?
diff --git a/activerecord/lib/arel/visitors/oracle12.rb b/activerecord/lib/arel/visitors/oracle12.rb
index b092aa95e0..9a7fe4d626 100644
--- a/activerecord/lib/arel/visitors/oracle12.rb
+++ b/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)
diff --git a/activerecord/lib/arel/visitors/to_sql.rb b/activerecord/lib/arel/visitors/to_sql.rb
index f9fe4404eb..b5a960ce68 100644
--- a/activerecord/lib/arel/visitors/to_sql.rb
+++ b/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)
diff --git a/activerecord/test/cases/batches_test.rb b/activerecord/test/cases/batches_test.rb
index d21218a997..cf6e280898 100644
--- a/activerecord/test/cases/batches_test.rb
+++ b/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
diff --git a/activerecord/test/cases/counter_cache_test.rb b/activerecord/test/cases/counter_cache_test.rb
index 99d286dc52..cc4f86a0fb 100644
--- a/activerecord/test/cases/counter_cache_test.rb
+++ b/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
diff --git a/activerecord/test/cases/inheritance_test.rb b/activerecord/test/cases/inheritance_test.rb
index 3d3189900f..19655a2d38 100644
--- a/activerecord/test/cases/inheritance_test.rb
+++ b/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
diff --git a/activesupport/test/test_case_test.rb b/activesupport/test/test_case_test.rb
index 8698c66e6d..56cd2665e0 100644
--- a/activesupport/test/test_case_test.rb
+++ b/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
diff --git a/guides/rails_guides/generator.rb b/guides/rails_guides/generator.rb
index 48e90510e1..fd33c3f8a7 100644
--- a/guides/rails_guides/generator.rb
+++ b/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
diff --git a/railties/lib/rails/code_statistics.rb b/railties/lib/rails/code_statistics.rb
index 19d331ff30..09082282f3 100644
--- a/railties/lib/rails/code_statistics.rb
+++ b/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|
diff --git a/railties/lib/rails/generators/rails/plugin/plugin_generator.rb b/railties/lib/rails/generators/rails/plugin/plugin_generator.rb
index 239b3a5739..294c8a2609 100644
--- a/railties/lib/rails/generators/rails/plugin/plugin_generator.rb
+++ b/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