expand documentation on type conversion w.r.t. UnsafeCell

core/src/cell.rs

@@ -1811,14 +1811,19 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 ///
 /// [`.get_mut()`]: `UnsafeCell::get_mut`
 ///
-/// `UnsafeCell<T>` has the same in-memory representation as its inner type `T` if and only if
-/// the type `T` does not contain a [niche] (e.g. the type `Option<NonNull<u8>>` is typically
-/// 8 bytes large on 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes
-/// up 16 bytes of space). A consequence of this guarantee is that it is possible to convert
-/// between `T` and `UnsafeCell<T>` when `T` has no niches. However, it is only valid to obtain
-/// a `*mut T` pointer to the contents of a _shared_ `UnsafeCell<T>` through [`.get()`] or
-/// [`.raw_get()`]. A `&mut T` reference can be obtained by either dereferencing this pointer
-/// or by calling [`.get_mut()`] on an _exclusive_ `UnsafeCell<T>`, e.g.:
+/// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
+/// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
+/// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
+/// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
+/// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
+/// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
+/// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
+/// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
+/// order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
+/// thus this can cause distortions in the type size in these cases. Furthermore, it is only valid
+/// to obtain a `*mut T` pointer to the contents of a _shared_ `UnsafeCell<T>` through [`.get()`]
+/// or [`.raw_get()`]. A `&mut T` reference can be obtained by either dereferencing this pointer or
+/// by calling [`.get_mut()`] on an _exclusive_ `UnsafeCell<T>`, e.g.:
 ///
 /// ```rust
 /// use std::cell::UnsafeCell;