Deprecate usage of nil as route path

In Rails 4 these kind of routes used to work:

```ruby
scope '/*id', controller: :builds, as: :build do
  get action: :show
end
```

But since 1a830cbd830c7f80936dff7e3c8b26f60dcc371d, routes are only created for
paths specified as strings or symbols. Implicit `nil` paths are just ignored,
with no deprecation warnings or errors. Routes are simply not created. This come
as a surprise for people migrating to Rails 5, since the lack of logs or errors
makes hard to understand where the problem is.

This commit introduces a deprecation warning in case of path as `nil`, while
still allowing the route definition.

1 files changed, 6 insertions, 0 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index e2cf75da3a..73b4864e45 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -1562,6 +1562,12 @@ module ActionDispatch
             options  = path
             path, to = options.find { |name, _value| name.is_a?(String) }
 
+            if path.nil?
+              ActiveSupport::Deprecation.warn 'Omitting the route path is deprecated. '\
+                'Specify the path with a String or a Symbol instead.'
+              path = ''
+            end
+
             case to
             when Symbol
               options[:action] = to