diff --git a/actioncable/test/test_helper.rb b/actioncable/test/test_helper.rb
index 7b1d600eb4..9f90fd1e8c 100644
--- a/actioncable/test/test_helper.rb
+++ b/actioncable/test/test_helper.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "action_cable"
 require "active_support/testing/autorun"
 require "active_support/testing/method_call_assertions"
diff --git a/actionmailer/test/abstract_unit.rb b/actionmailer/test/abstract_unit.rb
index 75ad14adbe..5abed4b24f 100644
--- a/actionmailer/test/abstract_unit.rb
+++ b/actionmailer/test/abstract_unit.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_support/core_ext/kernel/reporting"
 
 # These are the normal settings that will be set up by Railties
diff --git a/actionpack/test/abstract_unit.rb b/actionpack/test/abstract_unit.rb
index a05adbed3f..f5e1daa751 100644
--- a/actionpack/test/abstract_unit.rb
+++ b/actionpack/test/abstract_unit.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
+
 $:.unshift File.expand_path("lib", __dir__)
 
 require "active_support/core_ext/kernel/reporting"
diff --git a/actionview/lib/action_view/template/handlers/builder.rb b/actionview/lib/action_view/template/handlers/builder.rb
index 2e1371c015..4ae350fbfa 100644
--- a/actionview/lib/action_view/template/handlers/builder.rb
+++ b/actionview/lib/action_view/template/handlers/builder.rb
@@ -7,7 +7,8 @@ class Builder
 
       def call(template, source)
         require_engine
-        "xml = ::Builder::XmlMarkup.new(indent: 2, target: output_buffer.raw);" \
+        # the double assignment is to silence "assigned but unused variable" warnings
+        "xml = xml = ::Builder::XmlMarkup.new(indent: 2, target: output_buffer.raw);" \
           "#{source};" \
           "output_buffer.to_s"
       end
diff --git a/actionview/test/abstract_unit.rb b/actionview/test/abstract_unit.rb
index 15e4c02244..18ab12a63c 100644
--- a/actionview/test/abstract_unit.rb
+++ b/actionview/test/abstract_unit.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
+
 $:.unshift File.expand_path("lib", __dir__)
 
 ENV["TMPDIR"] = File.expand_path("tmp", __dir__)
diff --git a/activejob/test/helper.rb b/activejob/test/helper.rb
index 2b1aac8875..353e6f6f7d 100644
--- a/activejob/test/helper.rb
+++ b/activejob/test/helper.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_job"
 require "support/job_buffer"
 
diff --git a/activemodel/lib/active_model/type/serialize_cast_value.rb b/activemodel/lib/active_model/type/serialize_cast_value.rb
index 57dd97dbf0..d06c26e6d1 100644
--- a/activemodel/lib/active_model/type/serialize_cast_value.rb
+++ b/activemodel/lib/active_model/type/serialize_cast_value.rb
@@ -6,6 +6,7 @@ module SerializeCastValue # :nodoc:
       def self.included(klass)
         unless klass.respond_to?(:included_serialize_cast_value)
           klass.singleton_class.attr_accessor :included_serialize_cast_value
+          klass.silence_redefinition_of_method(:itself_if_class_included_serialize_cast_value)
           klass.attr_reader :itself_if_class_included_serialize_cast_value
         end
         klass.included_serialize_cast_value = true
diff --git a/activemodel/test/cases/helper.rb b/activemodel/test/cases/helper.rb
index 8fdb117da0..c049f9496b 100644
--- a/activemodel/test/cases/helper.rb
+++ b/activemodel/test/cases/helper.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_model"
 
 # Show backtraces for deprecated behavior for quicker cleanup.
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 4060826854..2de5d21e9c 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -311,6 +311,7 @@ def initialize(...)
 
         @max_identifier_length = nil
         @type_map = nil
+        @raw_connection = nil
 
         @use_insert_returning = @config.key?(:insert_returning) ? self.class.type_cast_config_to_boolean(@config[:insert_returning]) : true
       end
diff --git a/activerecord/test/cases/adapters/postgresql/statement_pool_test.rb b/activerecord/test/cases/adapters/postgresql/statement_pool_test.rb
index 373c723c6f..f0c5dbc2d2 100644
--- a/activerecord/test/cases/adapters/postgresql/statement_pool_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/statement_pool_test.rb
@@ -8,6 +8,10 @@ module ActiveRecord
   module ConnectionAdapters
     class PostgreSQLAdapter < AbstractAdapter
       class InactivePgConnection
+        def initialize
+          @raw_connection = nil
+        end
+
         def query(*args)
           raise PG::Error
         end
diff --git a/activerecord/test/cases/enum_test.rb b/activerecord/test/cases/enum_test.rb
index 2ca1e961ec..8cf862e9b7 100644
--- a/activerecord/test/cases/enum_test.rb
+++ b/activerecord/test/cases/enum_test.rb
@@ -413,7 +413,7 @@ class EnumTest < ActiveRecord::TestCase
     e = assert_raises(ArgumentError) do
       Class.new(ActiveRecord::Base) do
         self.table_name = "books"
-        enum :status, {}
+        enum(:status, {}, **{})
       end
     end
 
diff --git a/activerecord/test/cases/test_case.rb b/activerecord/test/cases/test_case.rb
index 8a6ce32b44..b20231c4a2 100644
--- a/activerecord/test/cases/test_case.rb
+++ b/activerecord/test/cases/test_case.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_support"
 require "active_support/testing/autorun"
 require "active_support/testing/method_call_assertions"
diff --git a/activestorage/test/test_helper.rb b/activestorage/test/test_helper.rb
index 6f92e787c3..5cea4dcfea 100644
--- a/activestorage/test/test_helper.rb
+++ b/activestorage/test/test_helper.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
+
 ENV["RAILS_ENV"] ||= "test"
 require_relative "dummy/config/environment.rb"
 
diff --git a/activesupport/lib/active_support/testing/strict_warnings.rb b/activesupport/lib/active_support/testing/strict_warnings.rb
new file mode 100644
index 0000000000..9063960c1e
--- /dev/null
+++ b/activesupport/lib/active_support/testing/strict_warnings.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+$VERBOSE = true
+Warning[:deprecated] = true
+
+module RaiseWarnings
+  PROJECT_ROOT = File.expand_path("../../../../", __dir__)
+  ALLOWED_WARNINGS = Regexp.union(
+    /circular require considered harmful.*delayed_job/, # Bug in delayed job.
+
+    # Expected non-verbose warning emitted by Rails.
+    /Ignoring .*\.yml because it has expired/,
+    /Failed to validate the schema cache because/,
+  )
+  def warn(message, *)
+    super
+
+    return unless message.include?(PROJECT_ROOT)
+    return if ALLOWED_WARNINGS.match?(message)
+    return unless ENV["RAILS_STRICT_WARNINGS"] || ENV["CI"]
+
+    raise message
+  end
+  ruby2_keywords :warn if respond_to?(:ruby2_keywords, true)
+end
+
+Warning.singleton_class.prepend(RaiseWarnings)
diff --git a/activesupport/test/abstract_unit.rb b/activesupport/test/abstract_unit.rb
index 8cccea0b00..f55b224cf9 100644
--- a/activesupport/test/abstract_unit.rb
+++ b/activesupport/test/abstract_unit.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
+
 ORIG_ARGV = ARGV.dup
 
 require "bundler/setup"
diff --git a/activesupport/test/deprecation_test.rb b/activesupport/test/deprecation_test.rb
index 79d6d83cc1..79f80542d3 100644
--- a/activesupport/test/deprecation_test.rb
+++ b/activesupport/test/deprecation_test.rb
@@ -779,16 +779,19 @@ def assert_callbacks_called_with(matchers = {})
         lambda do |*args|
           message, callstack, deprecator = args
           bindings << binding
+          [message, callstack, deprecator]
         end,
 
         lambda do |message, *other|
           callstack, deprecator = other
           bindings << binding
+          [callstack, deprecator]
         end,
 
         lambda do |message, callstack, *details|
           deprecation_horizon, gem_name = details
           bindings << binding
+          [deprecation_horizon, gem_name]
         end,
       ]
 
diff --git a/railties/lib/rails/command/base.rb b/railties/lib/rails/command/base.rb
index bb6d0893fe..547eb8c1a0 100644
--- a/railties/lib/rails/command/base.rb
+++ b/railties/lib/rails/command/base.rb
@@ -181,6 +181,7 @@ def namespaced_commands
         attr_reader :current_subcommand
 
         def invoke_command(command, *) # :nodoc:
+          @current_subcommand ||= nil
           original_subcommand, @current_subcommand = @current_subcommand, command.name
           super
         ensure
diff --git a/railties/test/abstract_unit.rb b/railties/test/abstract_unit.rb
index bb671c4953..1de8fbdc83 100644
--- a/railties/test/abstract_unit.rb
+++ b/railties/test/abstract_unit.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
+
 ENV["RAILS_ENV"] ||= "test"
 
 require "stringio"
diff --git a/railties/test/application/initializers/frameworks_test.rb b/railties/test/application/initializers/frameworks_test.rb
index efdef657d3..f77223a889 100644
--- a/railties/test/application/initializers/frameworks_test.rb
+++ b/railties/test/application/initializers/frameworks_test.rb
@@ -247,7 +247,9 @@ def show
         config.eager_load = true
       RUBY
 
-      require "#{app_path}/config/environment"
+      silence_warnings do
+        require "#{app_path}/config/environment"
+      end
       assert_not ActiveRecord::Base.connection.schema_cache.data_sources("posts")
     end
 
diff --git a/railties/test/application/zeitwerk_integration_test.rb b/railties/test/application/zeitwerk_integration_test.rb
index fc1d204a98..dc05689740 100644
--- a/railties/test/application/zeitwerk_integration_test.rb
+++ b/railties/test/application/zeitwerk_integration_test.rb
@@ -78,7 +78,7 @@ class RESTfulController < ApplicationController
     end
     RUBY
 
-    app_file "config/initializers/autoload_Y.rb", "Y"
+    app_file "config/initializers/autoload_Y.rb", "Y.succ"
 
     # Preconditions.
     assert_not Object.const_defined?(:X)
diff --git a/railties/test/configuration/middleware_stack_proxy_test.rb b/railties/test/configuration/middleware_stack_proxy_test.rb
index 6131cdd690..330af8910a 100644
--- a/railties/test/configuration/middleware_stack_proxy_test.rb
+++ b/railties/test/configuration/middleware_stack_proxy_test.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_support"
 require "active_support/testing/autorun"
 require "rails/configuration"
diff --git a/railties/test/generators/argv_scrubber_test.rb b/railties/test/generators/argv_scrubber_test.rb
index f72cb353e0..4f6d97b114 100644
--- a/railties/test/generators/argv_scrubber_test.rb
+++ b/railties/test/generators/argv_scrubber_test.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_support/test_case"
 require "active_support/testing/autorun"
 require "rails/generators/rails/app/app_generator"
diff --git a/railties/test/generators/generator_test.rb b/railties/test/generators/generator_test.rb
index 281d5a5572..6d5d89ba23 100644
--- a/railties/test/generators/generator_test.rb
+++ b/railties/test/generators/generator_test.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "active_support/testing/strict_warnings"
 require "active_support/test_case"
 require "active_support/testing/autorun"
 require "rails/generators/app_base"
diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index d6c9849345..2142f1fac3 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -8,6 +8,7 @@
 #
 # It is also good to know what is the bare minimum to get
 # Rails booted up.
+require "active_support/testing/strict_warnings"
 require "fileutils"
 require "shellwords"