Fix UB when dropping BoxBytes owning a zero-sized layout (#258)

src/allocation.rs

@@ -742,8 +742,9 @@ impl<I: ?Sized, T: ?Sized + TransparentWrapper<I>> TransparentWrapperAlloc<I>
 
 /// As `Box<[u8]>`, but remembers the original alignment.
 pub struct BoxBytes {
-  // SAFETY: `ptr` is owned, was allocated with `layout`, and points to
-  // `layout.size()` initialized bytes.
+  // SAFETY: `ptr` is owned, points to `layout.size()` initialized bytes, and
+  // was allocated with `layout` (unless `layout.size() == 0` in which case it
+  // is dangling).
   ptr: NonNull<u8>,
   layout: Layout,
 }
@@ -770,9 +771,12 @@ impl DerefMut for BoxBytes {
 
 impl Drop for BoxBytes {
   fn drop(&mut self) {
-    // SAFETY: See type invariant.
+    if self.layout.size() != 0 {
+      // SAFETY: See type invariant: if `self.layout.size() != 0`, then
+      // `self.ptr` is owned and was allocated with `self.layout`.
       unsafe { alloc::alloc::dealloc(self.ptr.as_ptr(), self.layout) };
     }
+  }
 }
 
 impl<T: ?Sized + sealed::BoxBytesOf> From<Box<T>> for BoxBytes {

tests/cast_slice_tests.rs

@@ -328,3 +328,17 @@ fn test_arc_slices() {
   let empty: Arc<[i8]> = cast_slice_arc::<(), i8>(Arc::new([(); 42]));
   assert!(empty.is_empty());
 }
+
+#[cfg(feature = "extern_crate_alloc")]
+#[test]
+fn box_bytes_zst() {
+  let x: BoxBytes = box_bytes_of(Box::new([0u8; 0]));
+  let _: Box<[u8]> = from_box_bytes(x);
+
+  let x: BoxBytes = box_bytes_of(Box::new([0u8; 0]));
+  let res: Result<Box<[()]>, _> = try_from_box_bytes(x);
+  assert_eq!(res.unwrap_err().0, PodCastError::SizeMismatch);
+
+  let x: BoxBytes = box_bytes_of(Box::new([(); 0]));
+  let _: Box<[u8]> = from_box_bytes(x);
+}