Add custom ArgumentError for invalid to: values

Previously, it was theoretically possible to define a route with a
Symbol as a `to:` value (or at least, it would not raise a
`NoMethodError`). However, passing a Symbol broke when `/#/.match?(to)`
was [replaced][1] with `to&.include?("#")` with the assumption that `to`
was always a String.

Instead of restoring the previous error, this commit improves how the
`to:` value is checked so that it raises an `ArgumentError` for any
invalid values. The extra strictness will specifically improve the error
when a Symbol or String that doesn't include a "#" are passed since they
were effectively equivalent to passing a `nil` value, or not specifying
`to:` at all.

[1]: 5726b1d

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -216,14 +216,23 @@ def normalize_options!(options, path_params, modyoule)
 
             if to.respond_to?(:action) || to.respond_to?(:call)
               options
-            else
-              to_endpoint = split_to to
-              controller  = to_endpoint[0] || default_controller
-              action      = to_endpoint[1] || default_action
+            elsif to.nil?
+              controller = default_controller
+              action = default_action
+
+              controller = add_controller_module(controller, modyoule)
+
+              options.merge! check_controller_and_action(path_params, controller, action)
+            elsif to.is_a?(String) && to.include?("#")
+              to_endpoint = to.split("#").map!(&:-@)
+              controller  = to_endpoint[0]
+              action      = to_endpoint[1]
 
               controller = add_controller_module(controller, modyoule)
 
               options.merge! check_controller_and_action(path_params, controller, action)
+            else
+              raise ArgumentError, ":to must respond_to? :action or :call, or it must be a String that includes '#'"
             end
           end
 
@@ -308,14 +317,6 @@ def check_part(name, part, path_params, hash)
             hash
           end
 
-          def split_to(to)
-            if to&.include?("#")
-              to.split("#").map!(&:-@)
-            else
-              []
-            end
-          end
-
           def add_controller_module(controller, modyoule)
             if modyoule && !controller.is_a?(Regexp)
               if controller&.start_with?("/")

actionpack/test/dispatch/routing_test.rb

@@ -4054,7 +4054,16 @@ def test_missing_controller_with_to
         get "/foo/bar", to: "foo"
       end
     }
-    assert_match(/Missing :controller/, ex.message)
+    assert_match(/:to must respond_to/, ex.message)
+  end
+
+  def test_to_is_a_symbol
+    ex = assert_raises(ArgumentError) {
+      draw do
+        get "/foo/bar", to: :foo
+      end
+    }
+    assert_match(/:to must respond_to/, ex.message)
   end
 
   def test_missing_action_on_hash