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

1 files changed, 19 insertions, 5 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