Add some optimizations

Author: daxpedda

library/core/src/cell/once.rs

@@ -126,8 +126,7 @@ 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`
@@ -211,10 +210,13 @@ 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.