1 files changed, 55 insertions, 17 deletions

diff --git a/actionpack/lib/action_controller/metal/implicit_render.rb b/actionpack/lib/action_controller/metal/implicit_render.rb
index 17fcc2fa02..6192fc0f9c 100644
--- a/actionpack/lib/action_controller/metal/implicit_render.rb
+++ b/actionpack/lib/action_controller/metal/implicit_render.rb
@@ -1,29 +1,62 @@
+require 'active_support/core_ext/string/strip'
+
 module ActionController
+  # Handles implicit rendering for a controller action that does not
+  # explicitly respond with +render+, +respond_to+, +redirect+, or +head+.
+  #
+  # For API controllers, the implicit response is always 204 No Content.
+  #
+  # For all other controllers, we use these heuristics to decide whether to
+  # render a template, raise an error for a missing template, or respond with
+  # 204 No Content:
+  #
+  # First, if we DO find a template, it's rendered. Template lookup accounts
+  # for the action name, locales, format, variant, template handlers, and more
+  # (see +render+ for details).
+  #
+  # Second, if we DON'T find a template but the controller action does have
+  # templates for other formats, variants, etc., then we trust that you meant
+  # to provide a template for this response, too, and we raise
+  # <tt>ActionController::UnknownFormat</tt> with an explanation.
+  #
+  # Third, if we DON'T find a template AND the request is a page load in a web
+  # browser (technically, a non-XHR GET request for an HTML response) where
+  # you reasonably expect to have rendered a template, then we raise
+  # <tt>ActionView::UnknownFormat</tt> with an explanation.
+  #
+  # Finally, if we DON'T find a template AND the request isn't a browser page
+  # load, then we implicitly respond with 204 No Content.
   module ImplicitRender
 
+    # :stopdoc:
     include BasicImplicitRender
 
-    # Renders the template corresponding to the controller action, if it exists.
-    # The action name, format, and variant are all taken into account.
-    # For example, the "new" action with an HTML format and variant "phone" 
-    # would try to render the <tt>new.html+phone.erb</tt> template.
-    #
-    # If no template is found <tt>ActionController::BasicImplicitRender</tt>'s implementation is called, unless
-    # a block is passed. In that case, it will override the super implementation.
-    #
-    #   default_render do
-    #     head 404 # No template was found
-    #   end
     def default_render(*args)
       if template_exists?(action_name.to_s, _prefixes, variants: request.variant)
         render(*args)
+      elsif any_templates?(action_name.to_s, _prefixes)
+        message = "#{self.class.name}\##{action_name} is missing a template " \
+          "for this request format and variant.\n" \
+          "\nrequest.formats: #{request.formats.map(&:to_s).inspect}" \
+          "\nrequest.variant: #{request.variant.inspect}"
+
+        raise ActionController::UnknownFormat, message
+      elsif interactive_browser_request?
+        message = "#{self.class.name}\##{action_name} is missing a template " \
+          "for this request format and variant.\n\n" \
+          "request.formats: #{request.formats.map(&:to_s).inspect}\n" \
+          "request.variant: #{request.variant.inspect}\n\n" \
+          "NOTE! For XHR/Ajax or API requests, this action would normally " \
+          "respond with 204 No Content: an empty white screen. Since you're " \
+          "loading it in a web browser, we assume that you expected to " \
+          "actually render a template, not… nothing, so we're showing an " \
+          "error to be extra-clear. If you expect 204 No Content, carry on. " \
+          "That's what you'll get from an XHR or API request. Give it a shot."
+
+        raise ActionController::UnknownFormat, message
       else
-        if block_given?
-          yield(*args)
-        else
-          logger.info "No template found for #{self.class.name}\##{action_name}, rendering head :no_content" if logger
-          super
-        end
+        logger.info "No template found for #{self.class.name}\##{action_name}, rendering head :no_content" if logger
+        super
       end
     end
 
@@ -32,5 +65,10 @@ module ActionController
         "default_render"
       end
     end
+
+    private
+      def interactive_browser_request?
+        request.get? && request.format == Mime[:html] && !request.xhr?
+      end
   end
 end