Brush up errors of `ActionDispatch::Routing::Mapper#mount`

* Integrate to raise `ArgumentError`
* Detailed error message when `path` is not defined
* Add a test case, invalid rack app is passed

1 files changed, 9 insertions, 6 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 7c0404ca62..e676c837b4 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -600,17 +600,20 @@ module ActionDispatch
         def mount(app, options = nil)
           if options
             path = options.delete(:at)
-          else
-            unless Hash === app
-              raise ArgumentError, "must be called with mount point"
-            end
-
+          elsif Hash === app
             options = app
             app, path = options.find { |k, _| k.respond_to?(:call) }
             options.delete(app) if app
           end
 
-          raise "A rack application must be specified" unless path
+          raise ArgumentError, "A rack application must be specified" unless app.respond_to?(:call)
+          raise ArgumentError, <<-MSG.strip_heredoc unless path
+            Must be called with mount point
+
+              mount SomeRackApp, at: "some_route"
+              or
+              mount(SomeRackApp => "some_route")
+          MSG
 
           rails_app = rails_app? app
           options[:as] ||= app_name(app, rails_app)