Add MultiDecorator variant to SyntaxExtension enum

src/libsyntax/ext/base.rs

@@ -418,11 +418,15 @@ impl MacResult for DummyResult {
 
 /// An enum representing the different kinds of syntax extensions.
 pub enum SyntaxExtension {
+    /// A syntax extension that is attached to an item and creates new items
+    /// based upon it.
+    Decorator(Box<ItemDecorator + 'static>),
+
     /// A syntax extension that is attached to an item and creates new items
     /// based upon it.
     ///
     /// `#[derive(...)]` is an `ItemDecorator`.
-    Decorator(Box<ItemDecorator + 'static>),
+    MultiDecorator(Box<MultiItemDecorator + 'static>),
 
     /// A syntax extension that is attached to an item and modifies it
     /// in-place.