1use core::cell::UnsafeCell;
2
3pub struct LazyCell<T> {
4 contents: UnsafeCell<Option<T>>,
5}
6impl<T> LazyCell<T> {
7 pub fn new() -> LazyCell<T> {
8 LazyCell {
9 contents: UnsafeCell::new(None),
10 }
11 }
12
13 pub fn borrow(&self) -> Option<&T> {
14 unsafe { &*self.contents.get() }.as_ref()
15 }
16
17 pub fn borrow_with(&self, closure: impl FnOnce() -> T) -> &T {
18 // First check if we're already initialized...
19 let ptr = self.contents.get();
20 if let Some(val) = unsafe { &*ptr } {
21 return val;
22 }
23 // Note that while we're executing `closure` our `borrow_with` may
24 // be called recursively. This means we need to check again after
25 // the closure has executed. For that we use the `get_or_insert`
26 // method which will only perform mutation if we aren't already
27 // `Some`.
28 let val = closure();
29 unsafe { (*ptr).get_or_insert(val) }
30 }
31}
32