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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)
}
}
}
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