fix: do not panic when there is an empty data field in MessageOut r…

…eceipt (#705)

* Do not panic when there is an empty data field in MessageOut receipt

* Address feedback; format quoted code

packages/fuel-indexer-macros/src/indexer.rs

@@ -105,6 +105,14 @@ fn process_fn_items(
                 }
             }
         })
+        .chain(
+            vec![quote! {
+                u64::MAX => {
+                    {}
+                }
+            }]
+            .into_iter(),
+        )
         .collect::<Vec<proc_macro2::TokenStream>>();
 
     let abi_type_decoders = abi_types
@@ -463,10 +471,25 @@ fn process_fn_items(
                                 }
                             }
                             Receipt::MessageOut { message_id, sender, recipient, amount, nonce, len, digest, data } => {
-                                let mut buf = [0u8; 8];
-                                buf.copy_from_slice(&data[0..8]);
-                                let type_id = u64::from_be_bytes(buf);
-                                let receipt = abi::MessageOut{ message_id, sender, recipient, amount, nonce, len, digest, data: data[8..].to_vec() };
+                                // It's possible that the data field was generated from an empty Sway `Bytes` array
+                                // in the send_message() instruction in which case the data field in the receipt will
+                                // have no type information or data to decode, so we decode an empty vector to a unit struct
+                                let type_id = data
+                                    .get(..8)
+                                    .map(|buffer| {
+                                        u64::from_be_bytes(
+                                            <[u8; 8]>::try_from(&buffer[..])
+                                                .expect("Could not get type ID for data in MessageOut receipt"),
+                                        )
+                                    })
+                                    .unwrap_or(u64::MAX);
+
+                                let data = data
+                                    .get(8..)
+                                    .map(|buffer| buffer.to_vec())
+                                    .unwrap_or(Vec::<u8>::new());
+
+                                let receipt = abi::MessageOut{ message_id, sender, recipient, amount, nonce, len, digest, data };
                                 decoder.decode_messageout(type_id, receipt);
                             }
                             Receipt::ScriptResult { result, gas_used } => {