What guarantees? Nothing prevents a function receiving a nursery from not using it. Unless the language can enforce it you don't really gain anything valuable over returning a Promise, except more complex code that doesn't compose.
> Much like you know that control flow will come back to the function you're reading after calling another function.
What about continuations? Exceptions? Aborts? setjmp()? I can think of many cases where control doesn't return to the caller that are perfectly valid.
Basically, either the code is so simple theres obviously no bugs, or the code is so complex theres no obvious bugs. I feel a nursery is closer to the later than the former.
