> So first... that's not how nursery.start_soon works :-). It actually takes an async function, not a coroutine object. [..]

Interesting, and I think I like it.  I definitely understand the motivation to not tell users the difference between coroutine functions and coroutine objects, but yeah, the ship has sailed for asyncio/curio/twisted.

Anyways, I'm not saying we shouldn't implement your proposal, even if it is only going to be used by Trio.  I'm saying that we need to consider all alternatives, and for that, I'd like to see what happens if we implement a tiny&fast coro wrapper in Trio.  Maybe the runtime overhead of it will be negligible.