Add const default for OnceCell and OnceLock

Signed-off-by: tison <[email protected]>

Author: tisonkun

library/core/src/cell/once.rs

@@ -353,7 +353,8 @@ impl<T> OnceCell<T> {
 }
 
 #[stable(feature = "once_cell", since = "1.70.0")]
-impl<T> Default for OnceCell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+impl<T> const Default for OnceCell<T> {
     #[inline]
     fn default() -> Self {
         Self::new()

library/std/src/sync/once_lock.rs

@@ -582,7 +582,8 @@ impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T> {}
 impl<T: UnwindSafe> UnwindSafe for OnceLock<T> {}
 
 #[stable(feature = "once_cell", since = "1.70.0")]
-impl<T> Default for OnceLock<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+impl<T> const Default for OnceLock<T> {
     /// Creates a new uninitialized cell.
     ///
     /// # Example