You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: once_cell.rs
    path of file: ./repos/tokio/tokio/src/util
    
    the code of the file until where you have to start completion: #![allow(dead_code)]
use std::cell::UnsafeCell;
use std::mem::MaybeUninit;
use std::sync::Once;

pub(crate) struct OnceCell<T> {
    once: Once,
    value: UnsafeCell<MaybeUninit<T>>,
}

unsafe impl<T: Send + Sync> Send for OnceCell<T> {}
unsafe impl<T: Send + Sync> Sync for OnceCell<T> {}

impl<T> OnceCell<T> {
    pub(crate) const fn new() -> Self {
        Self {
            once: Once::new(),
            value: UnsafeCell::new(MaybeU
    
    fn do_init(&self, init: impl FnOnce() -> T) {
        let value_ptr = self.value.get() as *mut T;

        self.once.call_once(|| {
            let set_to = init();

            // Safety: The `std::sync::Once` guarantees that this initialization
            // will run at most once, and that no thread can get past the
            // `call_once` until it has run exactly once. Thus, we have
            // exclusive access to `value`.
            unsafe {
                std::ptr::write(value_ptr, set_to);
            }
        });
    }