Fix UB when dropping BoxBytes owning a zero-sized layout

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,
 }
@@ -770,8 +771,11 @@ impl DerefMut for BoxBytes {
 
 impl Drop for BoxBytes {
   fn drop(&mut self) {
-    // SAFETY: See type invariant.
-    unsafe { alloc::alloc::dealloc(self.ptr.as_ptr(), self.layout) };
+    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) };
+    }
   }
 }
 
@@ -846,13 +850,18 @@ impl<T: AnyBitPattern> sealed::FromBoxBytes for [T] {
     let single_layout = Layout::new::<T>();
     if bytes.layout.align() != single_layout.align() {
       Err((PodCastError::AlignmentMismatch, bytes))
-    } else if single_layout.size() == 0 {
-      Err((PodCastError::SizeMismatch, bytes))
-    } else if bytes.layout.size() % single_layout.size() != 0 {
+    } else if (single_layout.size() == 0 && bytes.layout.size() != 0)
+      || (single_layout.size() != 0
+        && bytes.layout.size() % single_layout.size() != 0)
+    {
       Err((PodCastError::OutputSliceWouldHaveSlop, bytes))
     } else {
       let (ptr, layout) = bytes.into_raw_parts();
-      let length = layout.size() / single_layout.size();
+      let length = if single_layout.size() != 0 {
+        layout.size() / single_layout.size()
+      } else {
+        0
+      };
       let ptr =
         core::ptr::slice_from_raw_parts_mut(ptr.as_ptr() as *mut T, length);
       // SAFETY: See BoxBytes type invariant.

tests/cast_slice_tests.rs

@@ -328,3 +328,16 @@ 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 _: Box<[()]> = from_box_bytes(x);
+
+  let x: BoxBytes = box_bytes_of(Box::new([(); 0]));
+  let _: Box<[u8]> = from_box_bytes(x);
+}