New rule: don't return inside a nursery, nor break or continue an outer loop

[Zac-HD]

See python-trio/trio#1493 - I'm not sure whether this is a good idea for a lint rule, but wanted to track it so I can decide later.

[Zac-HD]

Per python-trio/trio#1493 (comment), let's do it!

I also like linting against break or continue for a loop outside the nursery, and think it makes sense to include that under the same rule.

[jakkdl]

This also applies to anyio, which appears to behave identically to trio regardless of backend.

I don't think it applies to asyncio, or at least I'm not able to manifest any counterintuitive behavior. This is largely because you cannot manually cancel their cancelscope-equivalents, and asyncio.Cancelled will always be transformed to a TimeoutError, so whenever you're losing the return value it's because you're getting an exception thrown in your face.

[oremanj]

I think this is still fairly hazardous:

async with asyncio.TaskGroup() as tg:
    tg.create_task(do_something_forever())
    await asyncio.sleep(5)
    return "all done!"

The return value won't get lost per se, but one does tend to expect a return statement to result in the caller being resumed promptly. Here nothing happens until do_something_forever() completes or is cancelled.