Create MissingExactTemplate exception with separate template

actionpack/lib/action_controller/metal/exceptions.rb

@@ -50,4 +50,7 @@ module ActionController
 
   class UnknownFormat < ActionControllerError #:nodoc:
   end
+
+  class MissingExactTemplate < UnknownFormat #:nodoc:
+  end
 end

actionpack/lib/action_controller/metal/implicit_render.rb

@@ -41,18 +41,8 @@ module ActionController
 
         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
+        message = "#{self.class.name}\##{action_name} is missing a template for request formats: #{request.formats.map(&:to_s).join(',')}"
+        raise ActionController::MissingExactTemplate, message
       else
         logger.info "No template found for #{self.class.name}\##{action_name}, rendering head :no_content" if logger
         super

actionpack/lib/action_dispatch/middleware/exception_wrapper.rb

@@ -12,6 +12,7 @@ module ActionDispatch
       "ActionController::UnknownHttpMethod"          => :method_not_allowed,
       "ActionController::NotImplemented"             => :not_implemented,
       "ActionController::UnknownFormat"              => :not_acceptable,
+      "ActionController::MissingExactTemplate"       => :not_acceptable,
       "ActionController::InvalidAuthenticityToken"   => :unprocessable_entity,
       "ActionController::InvalidCrossOriginRequest"  => :unprocessable_entity,
       "ActionDispatch::Http::Parameters::ParseError" => :bad_request,
@@ -26,7 +27,8 @@ module ActionDispatch
       "ActionController::RoutingError"         => "routing_error",
       "AbstractController::ActionNotFound"     => "unknown_action",
       "ActiveRecord::StatementInvalid"         => "invalid_statement",
-      "ActionView::Template::Error"         => "template_error"
+      "ActionView::Template::Error"            => "template_error",
+      "ActionController::MissingExactTemplate" => "missing_exact_template",
     )
 
     attr_reader :backtrace_cleaner, :exception, :line_number, :file

actionpack/lib/action_dispatch/middleware/templates/rescues/missing_exact_template.html.erb

@@ -0,0 +1,19 @@
+<header>
+  <h1>No template for interactive request</h1>
+</header>
+
+<div id="container">
+  <h3><%= h @exception.message %></h3>
+
+  <p class="summary">
+    <strong>NOTE!</strong><br>
+    Unless told otherwise, Rails expects an action to render a template with the same name,<br>
+    contained in a folder named after its controller.
+
+    If this controller is an API responding with 204 (No Content), <br>
+    which does not require a template,
+    then this error will occur when trying to access it via browser,<br>
+    since we expect an HTML template
+    to be rendered for such requests. If that's the case, carry on.
+  </p>
+</div>

actionpack/lib/action_dispatch/middleware/templates/rescues/missing_exact_template.text.erb

@@ -0,0 +1,3 @@
+Missing exact template
+
+<%= @exception.message %>

actionpack/test/controller/mime/respond_to_test.rb

@@ -658,13 +658,13 @@ class RespondToControllerTest < ActionController::TestCase
   end
 
   def test_variant_without_implicit_rendering_from_browser
-    assert_raises(ActionController::UnknownFormat) do
+    assert_raises(ActionController::MissingExactTemplate) do
       get :variant_without_implicit_template_rendering, params: { v: :does_not_matter }
     end
   end
 
   def test_variant_variant_not_set_and_without_implicit_rendering_from_browser
-    assert_raises(ActionController::UnknownFormat) do
+    assert_raises(ActionController::MissingExactTemplate) do
       get :variant_without_implicit_template_rendering
     end
   end

actionpack/test/controller/render_test.rb

@@ -650,7 +650,7 @@ class ImplicitRenderTest < ActionController::TestCase
   tests ImplicitRenderTestController
 
   def test_implicit_no_content_response_as_browser
-    assert_raises(ActionController::UnknownFormat) do
+    assert_raises(ActionController::MissingExactTemplate) do
       get :empty_action
     end
   end