diff --git a/core/src/future/future.rs b/core/src/future/future.rs
index f965afc8a5937..86963b548b9c6 100644
--- a/core/src/future/future.rs
+++ b/core/src/future/future.rs
@@ -35,7 +35,7 @@ use crate::task::{Context, Poll};
 pub trait Future {
     /// The type of value produced on completion.
     #[stable(feature = "futures_api", since = "1.36.0")]
-    #[rustc_diagnostic_item = "FutureOutput"]
+    #[cfg_attr(not(bootstrap), lang = "future_output")]
     type Output;
 
     /// Attempt to resolve the future to a final value, registering