Currently, the execute_message is taking just an operation: Vec<u8>.

There are other functions related to messages:

• ContractRuntime::message_id that returns an Option<MessageId>.
• ContractRuntime::message_is_bouncing that returns an Option<bool>.

The behavior is as follows:

• When the execution context is an execute_message, the results are some Some(...).
• In other contexts, it is a None.

This is rather cumbersome, and it would probably be better to have the following API:

fn execute_message(&mut self, is_bouncing: bool, message_id: MessageId, message: Vec<u8>) -> Result<(), ExecutionError>

and remove the functions message_is_bouncing and message_id from the ContractRuntime.

This is not a critical protocol change, but would make life easier for developers.