Add safety comments about Rc::get_unchecked_mut.

src/allocation.rs

@@ -327,6 +327,10 @@ pub fn cast_rc<A: NoUninit, B: AnyBitPattern>(input: Rc<A>) -> Rc<B> {
 ///
 /// On failure you get back an error along with the starting `Rc`.
 ///
+/// Safety: You must not use `Rc::get_mut_unchecked` on any `Rc` to the same
+/// allocation while any `Rc` to a type which is not `AnyBitPattern + NoUninit`
+/// exists.
+///
 /// ## Failure
 ///
 /// * The start and end content type of the `Rc` must have the exact same
@@ -357,6 +361,10 @@ pub fn cast_arc<A: NoUninit, B: AnyBitPattern>(input: Arc<A>) -> Arc<B> {
 ///
 /// On failure you get back an error along with the starting `Arc`.
 ///
+/// Safety: You must not use `Arc::get_mut_unchecked` on any `Arc` to the same
+/// allocation while any `Arc` to a type which is not `AnyBitPattern + NoUninit`
+/// exists.
+///
 /// ## Failure
 ///
 /// * The start and end content type of the `Arc` must have the exact same
@@ -387,6 +395,10 @@ pub fn cast_slice_rc<A: NoUninit, B: AnyBitPattern>(input: Rc<[A]>) -> Rc<[B]> {
 ///
 /// On failure you get back an error along with the starting `Rc<[T]>`.
 ///
+/// Safety: You must not use `Rc::get_mut_unchecked` on any `Rc` to the same
+/// allocation while any `Rc` to a type which is not `AnyBitPattern + NoUninit`
+/// exists.
+///
 /// ## Failure
 ///
 /// * The start and end content type of the `Rc<[T]>` must have the exact same
@@ -439,6 +451,10 @@ pub fn cast_slice_arc<A: NoUninit, B: AnyBitPattern>(
 ///
 /// On failure you get back an error along with the starting `Arc<[T]>`.
 ///
+/// Safety: You must not use `Arc::get_mut_unchecked` on any `Arc` to the same
+/// allocation while any `Arc` to a type which is not `AnyBitPattern + NoUninit`
+/// exists.
+///
 /// ## Failure
 ///
 /// * The start and end content type of the `Arc<[T]>` must have the exact same