diff --git a/xtask/src/codegen/rust.ungram b/xtask/src/codegen/rust.ungram
index 48d4f4b4a0..b653c14a7f 100644
--- a/xtask/src/codegen/rust.ungram
+++ b/xtask/src/codegen/rust.ungram
@@ -103,7 +103,7 @@ Enum =
   VariantList
 
 VariantList =
- '{' Variant* '}'
+ '{' (Variant (',' Variant)* ','?)? '}'
 
 Variant =
   Attr* Visibility? Name FieldList ('=' Expr)?