mock stability attribute

Signed-off-by: tison <wander4096@gmail.com>

Author: tisonkun

library/core/src/cell/once.rs

@@ -165,6 +165,7 @@ impl<T> OnceCell<T> {
     }
 
     #[inline]
+    #[stable(feature = "once_cell", since = "1.70.0")]
     pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut T
     where
         F: FnOnce() -> T,
@@ -210,10 +211,10 @@ impl<T> OnceCell<T> {
         if let Some(val) = self.get() {
             return Ok(val);
         }
-        self.try_init(f)?;
-        Ok(self.get().unwrap())
+        self.try_init(f)
     }
 
+    #[unstable(feature = "once_cell_try", issue = "109737")]
     pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
     where
         F: FnOnce() -> Result<T, E>,
@@ -228,7 +229,7 @@ impl<T> OnceCell<T> {
     // Avoid inlining the initialization closure into the common path that fetches
     // the already initialized value
     #[cold]
-    fn try_init<F, E>(&self, f: F) -> Result<(), E>
+    fn try_init<F, E>(&self, f: F) -> Result<&T, E>
     where
         F: FnOnce() -> Result<T, E>,
     {