guarantee the memory layout of Cell

Author: madsmtm

library/core/src/cell.rs

@@ -209,6 +209,10 @@ pub use once::OnceCell;
 
 /// A mutable memory location.
 ///
+/// # Memory layout
+///
+/// `Cell<T>` shares the same memory layout and caveats as `UnsafeCell<T>`.
+///
 /// # Examples
 ///
 /// In this example, you can see that `Cell<T>` enables mutation inside an