[Rusky]

"Not inert" does not at all imply "a single runtime within std+compiler." You've jumped way too far in the opposite direction there.

The problem is that the particular interface Rust chose for controlling dispatch is not granular enough. When you are doing your own dispatch, you only get access to separate tasks, but for individual futures you are at the mercy of combinators like `select!` or `FuturesUnordered` that only have a narrow view of the system.

A better design would continue to avoid heap allocations and allow you to do your own dispatch, but operate in terms of individual suspended leaf futures. Combinators like `join!`/`select!`/etc. would be implemented more like they are in thread-based systems, waiting for sub-tasks to complete, rather than being responsible for driving them.

[vlovich123]

If you’ve got eager dispatch I’m eager (pun intended) to learn how you have an executor that’s not baked into the std library and limited to a single runtime per process because at the time of construction you need the language to schedule dispatch of the created future. This is one of the main challenges behind the pluggable executor effort - the set of executors that could be written is so different (work stealing vs thread per core) that it’s impossible to unify without an effect system and even then you’ve got challenges of how to encode that in the language structure because the executor is a global thing determined at runtime but then it’s also local in the sense that you don’t know which executor a given piece of code will end up actually being dispatched into since you could have the same async function invoked on different executors.

For better or worse eager dispatch I think generally implies also not being able to cancel futures since ownership is transferred to the executor rather than being retained by your code.

[Rusky]

You don't need any of that, and you can keep cancellation too.

The core of an eager cooperative multitasking system does not even need the concept of an executor. You can spawn a new task by giving it some stack space and running its body to its first suspension point, right there on the current thread. When it suspends, the leaf API (e.g. `lock`) grabs the current top of the stack and stashes it somewhere, and when it's time to resume it again just runs the next part of the task right there on the current thread.

You can build different kinds of schedulers on top of this first-class ability to resume a particular leaf call in a task. For example, a `lock` integrated with a particular scheduler might queue up the resume somewhere instead of invoking it immediately. Or, a generic `lock` might be wrapped with an adapter that re-suspends and queues that up. None of this requires that the language know anything about the scheduler at all.

This is all typical of how higher level languages implement both stackful and stackless coroutines. The difference is that we want control over the "give it some stack space" part- we want the compiler to compute a maximum size and have us specify where to store it, whether that's on the heap (e.g. tokio::spawn) or nested in some other task's stack (e.g. join, select) or some statically-allocated storage (e.g. on a microcontroller).

(Of course the question then becomes, how do you ensure `lock` can't resume the task after it's been freed, either due to normal resumption or cancellation? Rust answers this with `Waker`, but this conflates the unit of stack ownership with the unit of scheduling, and in the process enables intermediate futures to route a given wakeup incorrectly. These must be decoupled so that `lock` can hold onto both the overall stack and the exact leaf suspension point it will eventually resume.)

Cancellation doesn't change much here. Given a task held from the "caller end" (as opposed to the leaf callee resume handles above), the language needs to provide a way to destruct the stack and let the decoupled `Waker` mechanism respond. This still propagates naturally to nested tasks like join/select arms, though there is now an additional wrinkle that a nested task may be actively running (and may even be the thing that indirectly provoked the cancellation).