Because there is no “instead”? A promise (or really a task if you’re in an async context, what you link to is really a thread + a pseudo-oneshot channel) allows independently scheduling a unit of work, it’s neither shared nor mutable state.
The promise object you link to is not Clone, so there is no way for two different threads to refer to the same one without adding an intermediate `Arc`, and because all the methods on it operate by value that’s not exactly helpful either as you can’t move the promise out of the Arc.
Same with a tokio::task::Task. A JoinHandle can’t be duplicated (though according to its doc on some platforms it might be duplicatable — if the thread returned a duplicatable value anyway).
The promise object you link to is not Clone, so there is no way for two different threads to refer to the same one without adding an intermediate `Arc`, and because all the methods on it operate by value that’s not exactly helpful either as you can’t move the promise out of the Arc.
Same with a tokio::task::Task. A JoinHandle can’t be duplicated (though according to its doc on some platforms it might be duplicatable — if the thread returned a duplicatable value anyway).