Why does spawn for a single thread runtime require Send ?

[tsuberim]

I have a non-Send type that I want to pass across await, I know that this is impossible when we cross thread boundaries but why should it be impossible when I specifically created a single-threaded runtime using new_current_thread(..) ?

[Darksonn]

The Runtime object itself is Send, so you can still send it across threads. Use a LocalSet if you want non-Send tasks.

[tsuberim]

Hmm... because the runtime keeps a state of the current futures? So if a future contains a ref to a non-send that makes the entire Runtime non-sendable. Is this correct?

Maybe we can provide a special SingleThreadRuntime without those restrictions.

[Noah-Kennedy]

The runtime is Send and Sync to enable you to do things like access it from multiple threads simultaneously. LocalSet provides a way to have !Send futures in the root task. This works because the root task is entered into via block_on, so the runtime can't be sent while a localset is in operation.

[Darksonn]

Maybe we can provide a special SingleThreadRuntime without those restrictions.

We do, and it's called LocalSet.