Document requirements for unsized {Rc,Arc}::from_raw

library/alloc/src/rc.rs

@@ -1181,12 +1181,19 @@ impl<T: ?Sized> Rc<T> {
     /// Constructs an `Rc<T>` from a raw pointer.
     ///
     /// The raw pointer must have been previously returned by a call to
-    /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
-    /// and alignment as `T`. This is trivially true if `U` is `T`.
-    /// Note that if `U` is not `T` but has the same size and alignment, this is
-    /// basically like transmuting references of different types. See
-    /// [`mem::transmute`][transmute] for more information on what
-    /// restrictions apply in this case.
+    /// [`Rc<U>::into_raw`][into_raw] with the following requirements:
+    /// 
+    /// * If `U` is sized, it must have the same size and alignment as `T`. This
+    ///   is trivially true if `U` is `T`.
+    /// * If `U` is unsized, its data pointer must have the same size and
+    ///   alignment as `T`. This is trivially true if `Arc<U>` was constructed
+    ///   through `Arc<T>` and then converted to `Arc<U>` through an [unsized
+    ///   coercion].
+    ///
+    /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
+    /// and alignment, this is basically like transmuting references of
+    /// different types. See [`mem::transmute`][transmute] for more information
+    /// on what restrictions apply in this case.
     ///
     /// The raw pointer must point to a block of memory allocated by the global allocator
     ///
@@ -1198,6 +1205,7 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// [into_raw]: Rc::into_raw
     /// [transmute]: core::mem::transmute
+    /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
     ///
     /// # Examples
     ///

library/alloc/src/sync.rs

@@ -1329,12 +1329,19 @@ impl<T: ?Sized> Arc<T> {
     /// Constructs an `Arc<T>` from a raw pointer.
     ///
     /// The raw pointer must have been previously returned by a call to
-    /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
-    /// alignment as `T`. This is trivially true if `U` is `T`.
-    /// Note that if `U` is not `T` but has the same size and alignment, this is
-    /// basically like transmuting references of different types. See
-    /// [`mem::transmute`][transmute] for more information on what
-    /// restrictions apply in this case.
+    /// [`Arc<U>::into_raw`][into_raw] with the following requirements:
+    /// 
+    /// * If `U` is sized, it must have the same size and alignment as `T`. This
+    ///   is trivially true if `U` is `T`.
+    /// * If `U` is unsized, its data pointer must have the same size and
+    ///   alignment as `T`. This is trivially true if `Arc<U>` was constructed
+    ///   through `Arc<T>` and then converted to `Arc<U>` through an [unsized
+    ///   coercion].
+    ///
+    /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
+    /// and alignment, this is basically like transmuting references of
+    /// different types. See [`mem::transmute`][transmute] for more information
+    /// on what restrictions apply in this case.
     ///
     /// The user of `from_raw` has to make sure a specific value of `T` is only
     /// dropped once.
@@ -1344,6 +1351,7 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// [into_raw]: Arc::into_raw
     /// [transmute]: core::mem::transmute
+    /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
     ///
     /// # Examples
     ///