Update to quote 0.4
diff --git a/src/expr.rs b/src/expr.rs
index 3f18fa4..a7c8c33 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -639,7 +639,7 @@
         assert!(index < std::u32::MAX as usize);
         Index {
             index: index as u32,
-            span: Span::default(),
+            span: Span::def_site(),
         }
     }
 }