Function is_cancel_request_notification 

pub fn is_cancel_request_notification<N: JsonRpcNotification>(
    notification: &N,
) -> bool

Available on crate feature unstable_cancel_request only.

Whether a notification is a $/cancel_request, even when it is still wrapped in _proxy/successor envelopes.

$/cancel_request is connection-scoped: its requestId was allocated on the connection the notification arrived over and means nothing on any other connection. Generic forwarding code (such as ConnectionTo::send_proxied_message_to) uses this check to drop the raw notification instead of tunneling it across a hop; the cancellation still propagates because forward_response_to re-issues it with the forwarded request’s own ID.

Checking a notification whose method is not the successor envelope is a plain method-name comparison. Only successor-wrapped notifications pay for a serialization to peel the envelope.