Move require_dependencies suite to the one for dependencies

require_dependency has now one single definition, and its implementation
does not depend on Zeitwerk or even applications. It only depends on
having some autoload paths in place.

We can test that in the AS test suite.

activesupport/lib/active_support/dependencies/zeitwerk_integration.rb

@@ -43,17 +43,6 @@ def unhook!
         end
       end
 
-      module RequireDependency
-        def require_dependency(filename)
-          filename = filename.to_path if filename.respond_to?(:to_path)
-          if abspath = ActiveSupport::Dependencies.search_for_file(filename)
-            require abspath
-          else
-            require filename
-          end
-        end
-      end
-
       module Inflector
         # Concurrent::Map is not needed. This is a private class, and overrides
         # must be defined while the application boots.
@@ -116,7 +105,6 @@ def freeze_paths
           def decorate_dependencies
             Dependencies.unhook!
             Dependencies.singleton_class.prepend(Decorations)
-            Object.prepend(RequireDependency)
           end
       end
     end

activesupport/test/dependencies_test.rb

@@ -114,6 +114,91 @@ def test_load_and_require_stay_private
   ensure
     ActiveSupport::Dependencies.hook!
   end
+end
+
+class RequireDependencyTest < ActiveSupport::TestCase
+  setup do
+    @loaded_features_copy = $LOADED_FEATURES.dup
+    ActiveSupport::Dependencies.autoload_paths.clear
+
+    @root_dir = Dir.mktmpdir
+    File.write("#{@root_dir}/x.rb", "X = :X")
+    ActiveSupport::Dependencies.autoload_paths << @root_dir
+  end
+
+  teardown do
+    $LOADED_FEATURES.replace(@loaded_features_copy)
+    ActiveSupport::Dependencies.autoload_paths.clear
+
+    FileUtils.rm_rf(@root_dir)
+    Object.send(:remove_const, :X) if Object.const_defined?(:X)
+  end
+
+  test "require_dependency looks autoload paths up" do
+    assert require_dependency("x")
+    assert_equal :X, X
+  end
+
+  test "require_dependency looks autoload paths up (idempotent)" do
+    assert  require_dependency("x")
+    assert !require_dependency("x")
+  end
+
+  test "require_dependency handles absolute paths correctly" do
+    assert require_dependency("#{@root_dir}/x.rb")
+    assert_equal :X, X
+  end
+
+  test "require_dependency handles absolute paths correctly (idempotent)" do
+    assert  require_dependency("#{@root_dir}/x.rb")
+    assert !require_dependency("#{@root_dir}/x.rb")
+  end
 
-  # Coverage for require_dependency can be found in the railties test suite.
+  test "require_dependency supports arguments that respond to to_path" do
+    x = Object.new
+    def x.to_path; "x"; end
+
+    assert require_dependency(x)
+    assert_equal :X, X
+  end
+
+  test "require_dependency supports arguments that respond to to_path (idempotent)" do
+    x = Object.new
+    def x.to_path; "x"; end
+
+    assert  require_dependency(x)
+    assert !require_dependency(x)
+  end
+
+  test "require_dependency fallback to Kernel#require" do
+    dir = Dir.mktmpdir
+    $LOAD_PATH << dir
+    File.write("#{dir}/y.rb", "Y = :Y")
+
+    assert require_dependency("y")
+    assert_equal :Y, Y
+  ensure
+    $LOAD_PATH.pop
+    Object.send(:remove_const, :Y) if Object.const_defined?(:Y)
+  end
+
+  test "require_dependency fallback to Kernel#require (idempotent)" do
+    dir = Dir.mktmpdir
+    $LOAD_PATH << dir
+    File.write("#{dir}/y.rb", "Y = :Y")
+
+    assert  require_dependency("y")
+    assert !require_dependency("y")
+  ensure
+    $LOAD_PATH.pop
+    Object.send(:remove_const, :Y) if Object.const_defined?(:Y)
+  end
+
+  test "require_dependency raises ArgumentError if the argument is not a String and does not respond to #to_path" do
+    assert_raises(ArgumentError) { require_dependency(Object.new) }
+  end
+
+  test "require_dependency raises LoadError if the given argument is not found" do
+    assert_raise(LoadError) { require_dependency("nonexistent_filename") }
+  end
 end

railties/test/application/zeitwerk_integration_test.rb

@@ -139,43 +139,6 @@ def self.name
     assert_empty deps.autoloaded_constants
   end
 
-  [true, false].each do |add_aps_to_lp|
-    test "require_dependency looks autoload paths up (#{add_aps_to_lp})" do
-      add_to_config "config.add_autoload_paths_to_load_path = #{add_aps_to_lp}"
-      app_file "app/models/user.rb", "class User; end"
-      boot
-
-      assert require_dependency("user")
-    end
-
-    test "require_dependency handles absolute paths correctly (#{add_aps_to_lp})" do
-      add_to_config "config.add_autoload_paths_to_load_path = #{add_aps_to_lp}"
-      app_file "app/models/user.rb", "class User; end"
-      boot
-
-      assert require_dependency("#{app_path}/app/models/user.rb")
-    end
-
-    test "require_dependency supports arguments that respond to to_path (#{add_aps_to_lp})" do
-      add_to_config "config.add_autoload_paths_to_load_path = #{add_aps_to_lp}"
-      app_file "app/models/user.rb", "class User; end"
-      boot
-
-      user = Object.new
-      def user.to_path; "user"; end
-
-      assert require_dependency(user)
-    end
-  end
-
-  test "require_dependency raises ArgumentError if the argument is not a String and does not respond to #to_path" do
-    assert_raises(ArgumentError) { require_dependency(Object.new) }
-  end
-
-  test "require_dependency raises LoadError if the given argument is not found" do
-    assert_raise(LoadError) { require_dependency("nonexistent_filename") }
-  end
-
   test "eager loading loads the application code" do
     $zeitwerk_integration_test_user = false
     $zeitwerk_integration_test_post = false