use core::cell::UnsafeCell;

pub struct LazyCell<T> {
    contents: UnsafeCell<Option<T>>,
}
impl<T> LazyCell<T> {
    pub fn new() -> LazyCell<T> {
        LazyCell {
            contents: UnsafeCell::new(None),
        }
    }

    pub fn borrow_with(&self, closure: impl FnOnce() -> T) -> &T {
        unsafe {
            // First check if we're already initialized...
            let ptr = self.contents.get();
            if let Some(val) = &*ptr {
                return val;
            }
            // Note that while we're executing `closure` our `borrow_with` may
            // be called recursively. This means we need to check again after
            // the closure has executed. For that we use the `get_or_insert`
            // method which will only perform mutation if we aren't already
            // `Some`.
            let val = closure();
            (*ptr).get_or_insert(val)
        }
    }
}