diff --git a/src/send-and-sync.md b/src/send-and-sync.md
index d11b118..4652a85 100644
--- a/src/send-and-sync.md
+++ b/src/send-and-sync.md
@@ -54,7 +54,7 @@ In the *incredibly rare* case that a type is inappropriately automatically
 derived to be Send or Sync, then one can also unimplement Send and Sync:
 
 ```rust
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 // I have some magic semantics for some synchronization primitive!
 struct SpecialThreadToken(u8);