Deprecate `with_fallbacks` using a block

This patch changes `with_fallbacks` to be a factory method that returns
a new instance of a lookup context which contains the fallback view
paths in addition to the controller specific view paths.  Since the
lookup context is more "read only", we may be able to cache them

3 files changed, 42 insertions, 15 deletions

diff --git a/actionview/lib/action_view/lookup_context.rb b/actionview/lib/action_view/lookup_context.rb
index cc262dc2b7..c2f6439e26 100644
--- a/actionview/lib/action_view/lookup_context.rb
+++ b/actionview/lib/action_view/lookup_context.rb
@@ -3,6 +3,7 @@
 require "concurrent/map"
 require "active_support/core_ext/module/remove_method"
 require "active_support/core_ext/module/attribute_accessors"
+require "active_support/deprecation"
 require "action_view/template/resolver"
 
 module ActionView
@@ -132,11 +133,24 @@ module ActionView
       # Adds fallbacks to the view paths. Useful in cases when you are rendering
       # a :file.
       def with_fallbacks
-        view_paths = @view_paths
-        @view_paths = build_view_paths((view_paths.paths + self.class.fallbacks).uniq)
-        yield
-      ensure
-        @view_paths = view_paths
+        view_paths = build_view_paths((@view_paths.paths + self.class.fallbacks).uniq)
+
+        if block_given?
+          ActiveSupport::Deprecation.warn <<~eowarn
+          Calling `with_fallbacks` with a block is deprecated.  Call methods on
+          the lookup context returned by `with_fallbacks` instead.
+          eowarn
+
+          begin
+            _view_paths = @view_paths
+            @view_paths = view_paths
+            yield
+          ensure
+            @view_paths = _view_paths
+          end
+        else
+          ActionView::LookupContext.new(view_paths, @details, @prefixes)
+        end
       end
 
     private
diff --git a/actionview/lib/action_view/renderer/template_renderer.rb b/actionview/lib/action_view/renderer/template_renderer.rb
index fb5539e0db..6a50ccff7f 100644
--- a/actionview/lib/action_view/renderer/template_renderer.rb
+++ b/actionview/lib/action_view/renderer/template_renderer.rb
@@ -28,7 +28,7 @@ module ActionView
         elsif options.key?(:html)
           Template::HTML.new(options[:html], formats.first)
         elsif options.key?(:file)
-          with_fallbacks { find_file(options[:file], nil, false, keys, @details) }
+          @lookup_context.with_fallbacks.find_file(options[:file], nil, false, keys, @details)
         elsif options.key?(:inline)
           handler = Template.handler_for_extension(options[:type] || "erb")
           Template.new(options[:inline], "inline template", handler, locals: keys)
@@ -36,7 +36,7 @@ module ActionView
           if options[:template].respond_to?(:render)
             options[:template]
           else
-            find_template(options[:template], options[:prefixes], false, keys, @details)
+            @lookup_context.find_template(options[:template], options[:prefixes], false, keys, @details)
           end
         else
           raise ArgumentError, "You invoked render but did not give any of :partial, :template, :inline, :file, :plain, :html or :body option."
@@ -82,9 +82,9 @@ module ActionView
         when String
           begin
             if layout.start_with?("/")
-              with_fallbacks { find_template(layout, nil, false, [], details) }
+              @lookup_context.with_fallbacks.find_template(layout, nil, false, [], details)
             else
-              find_template(layout, nil, false, [], details)
+              @lookup_context.find_template(layout, nil, false, [], details)
             end
           rescue ActionView::MissingTemplate
             all_details = @details.merge(formats: @lookup_context.default_formats)
diff --git a/actionview/test/template/lookup_context_test.rb b/actionview/test/template/lookup_context_test.rb
index 0894925c9f..290f832794 100644
--- a/actionview/test/template/lookup_context_test.rb
+++ b/actionview/test/template/lookup_context_test.rb
@@ -122,19 +122,32 @@ class LookupContextTest < ActiveSupport::TestCase
   test "adds fallbacks to view paths when required" do
     assert_equal 1, @lookup_context.view_paths.size
 
-    @lookup_context.with_fallbacks do
-      assert_equal 3, @lookup_context.view_paths.size
-      assert_includes @lookup_context.view_paths, ActionView::FallbackFileSystemResolver.new("")
-      assert_includes @lookup_context.view_paths, ActionView::FallbackFileSystemResolver.new("/")
+    assert_deprecated do
+      @lookup_context.with_fallbacks do
+        assert_equal 3, @lookup_context.view_paths.size
+        assert_includes @lookup_context.view_paths, ActionView::FallbackFileSystemResolver.new("")
+        assert_includes @lookup_context.view_paths, ActionView::FallbackFileSystemResolver.new("/")
+      end
     end
+
+    @lookup_context = @lookup_context.with_fallbacks
+
+    assert_equal 3, @lookup_context.view_paths.size
+    assert_includes @lookup_context.view_paths, ActionView::FallbackFileSystemResolver.new("")
+    assert_includes @lookup_context.view_paths, ActionView::FallbackFileSystemResolver.new("/")
   end
 
   test "add fallbacks just once in nested fallbacks calls" do
-    @lookup_context.with_fallbacks do
+    assert_deprecated do
       @lookup_context.with_fallbacks do
-        assert_equal 3, @lookup_context.view_paths.size
+        @lookup_context.with_fallbacks do
+          assert_equal 3, @lookup_context.view_paths.size
+        end
       end
     end
+
+    @lookup_context = @lookup_context.with_fallbacks.with_fallbacks
+    assert_equal 3, @lookup_context.view_paths.size
   end
 
   test "generates a new details key for each details hash" do