Rollup merge of rust-lang#143341 - Manishearth:from-raw-parts-ptr-cast, r=samueltardieu

Mention that casting to *const () is a way to roundtrip with from_raw_parts

See discussion on rust-lang#81513

Author: matthiaskrgr

core/src/ptr/metadata.rs

@@ -106,6 +106,9 @@ pub const fn metadata<T: PointeeSized>(ptr: *const T) -> <T as Pointee>::Metadat
 /// For slices, see the documentation of [`slice::from_raw_parts`] for safety requirements.
 /// For trait objects, the metadata must come from a pointer to the same underlying erased type.
 ///
+/// If you are attempting to deconstruct a DST in a generic context to be reconstructed later,
+/// a thin pointer can always be obtained by casting `*const T` to `*const ()`.
+///
 /// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
 #[unstable(feature = "ptr_metadata", issue = "81513")]
 #[inline]