fix bug in setting a data list to a null list

capnp/src/private/layout.rs

@@ -2032,7 +2032,10 @@ mod wire_helpers {
                 // in the canonicalize=true case.
                 let whole_byte_size =
                     u64::from(value.element_count) * u64::from(value.step) / BITS_PER_BYTE as u64;
-                ptr::copy_nonoverlapping(value.ptr, ptr, whole_byte_size as usize);
+                if whole_byte_size > 0 {
+                    ptr::copy_nonoverlapping(value.ptr, ptr, whole_byte_size as usize);
+                }
+
                 let leftover_bits =
                     u64::from(value.element_count) * u64::from(value.step) % BITS_PER_BYTE as u64;
                 if leftover_bits > 0 {

capnpc/test/test.rs

@@ -1525,6 +1525,21 @@ mod tests {
         }
     }
 
+    /// https://github.com/capnproto/capnproto-rust/issues/525
+    #[test]
+    fn set_data_list_null() {
+        use crate::test_capnp::test_all_types;
+
+        let mut message = message::Builder::new_default();
+        let root: test_all_types::Builder<'_> = message.init_root();
+
+        let mut message2 = message::Builder::new_default();
+        let mut root2: test_all_types::Builder<'_> = message2.init_root();
+        root2
+            .set_data_list(root.into_reader().get_data_list().unwrap())
+            .unwrap();
+    }
+
     #[test]
     fn double_far_pointer() {
         let segment0: &[capnp::Word] = &[