still accept references to u8 slices and str in packed fields

compiler/rustc_const_eval/src/util/alignment.rs

@@ -21,12 +21,18 @@ where
     };
 
     let ty = place.ty(local_decls, tcx).ty;
+    let unsized_tail = || tcx.struct_tail_with_normalize(ty, |ty| ty, || {});
     match tcx.layout_of(param_env.and(ty)) {
-        Ok(layout) if layout.align.abi <= pack && layout.is_sized() => {
+        Ok(layout)
+            if layout.align.abi <= pack
+                && (layout.is_sized()
+                    || matches!(unsized_tail().kind(), ty::Slice(..) | ty::Str)) =>
+        {
             // If the packed alignment is greater or equal to the field alignment, the type won't be
             // further disaligned.
             // However we need to ensure the field is sized; for unsized fields, `layout.align` is
-            // just an approximation.
+            // just an approximation -- except when the unsized tail is a slice, where the alignment
+            // is fully determined by the type.
             debug!(
                 "is_disaligned({:?}) - align = {}, packed = {}; not disaligned",
                 place,