The coap-message crate provides traits that describe readable and writable CoAP messages in different levels of complexity. They are built to be used in server and client applications as an interface between the CoAP library, transport, and the application. Using them, the business logic of reading an incoming request and populating the response (or vice versa in a client) can be largely independent of the underlying CoAP library or transport.
The traits in this crate only cover the properties of CoAP at the request/response exchange level, not of the transport specific details. In particular, they do not expose tokens, message IDs or message types: Those are not even present in some transports, and in general there is no need for libraries to expose them. In the same vein, no methods are described to allocate messages: For constrained devices, that may not even be possible, and with CoAP libraries that support multiple configured transports, the type of message allocated may easily depend on the intended use.