Add some optimizations

library/core/src/cell/once.rs

@@ -128,8 +128,7 @@ impl<T> OnceCell<T> {
  // checked that slot is currently `None`, so this write
  // maintains the `inner`'s invariant.
  let slot = unsafe { &mut *self.inner.get() };
- *slot = Some(value);
- Ok(self.get().unwrap())
+ Ok(slot.insert(value))
  }
 
  /// Gets the contents of the cell, initializing it with `f`
@@ -213,10 +212,9 @@ impl<T> OnceCell<T> {
  let val = outlined_call(f)?;
  // Note that *some* forms of reentrant initialization might lead to
  // UB (see `reentrant_init` test). I believe that just removing this
- // `assert`, while keeping `set/get` would be sound, but it seems
+ // `panic`, while keeping `try_insert` would be sound, but it seems
  // better to panic, rather than to silently use an old value.
- assert!(self.set(val).is_ok(), "reentrant init");
- Ok(self.get().unwrap())
+ if let Ok(val) = self.try_insert(val) { Ok(val) } else { panic!("reentrant init") }
  }
 
  /// Consumes the cell, returning the wrapped value.