Add `OnceCell<T>: !Sync` impl for diagnostics

diff --git a/library/core/src/cell/once.rs b/library/core/src/cell/once.rs
index 7757068a4f2b97cbdbf606c90078e1d0fc3925ff..f74e563f1b9c0bc5ef8de2f77929c36c4d5633f9 100644 (file)
--- a/library/core/src/cell/once.rs
+++ b/library/core/src/cell/once.rs
@@ -298,3 +298,7 @@ fn from(value: T) -> Self {
         OnceCell { inner: UnsafeCell::new(Some(value)) }
     }
 }
+
+// Just like for `Cell<T>` this isn't needed, but results in nicer error messages.
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T> !Sync for OnceCell<T> {}