Error on unused awaitable expressions

[jhance]

Generates an error when an expression has a type which has a
defined await in its MRO but is not used. This includes all
builtin awaitable objects (e.g. futures, coroutines, and tasks)
but also anything a user might define which is also awaitable.

A hint is attached to suggest awaiting.

This can be extended in the future to other types of values that
may lead to a resource leak if needed, or even exposed as a plugin.

[JelleZijlstra]

This one is a false positive: https://github.com/aiortc/aiortc/blob/c258ef4167eb04914646db03c6044ca05428893a/src/aiortc/rtcsctptransport.py#L1393. There's no need to await asyncio.ensure_future. And in any case, you can't add an await in a non-async function.

[jhance]

Failure to use a future likely indicates a resource leak bug because you won't cancel in the event of failure, so I wouldn't classify it as a false positive.

I think its reasonable that if you don't want to use the value you do _ =.

[jhance]

Also, docs explicitly say not to throw away the result of ensure_future or create_task: https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task

[JukkaL]

At least homeassistant seems to have some false positives. It has a async_create_task method that returns a Task, but it also records the task object so that actually won't be leaked, so the return value can be ignored.

I can suggest a few things we could do about these:

1. Only complain about unused Task (or generally non-coroutine) values from certain known asyncio methods whose return values should not be ignored.
2. Use different error codes for ignored coroutines and other awaitables. Only complain about coroutines by default, and the general awaitable errors would have to be explicitly enabled, since they can cause false positives.

In any case, it would be good to use a dedicated error code for these errors.

There's also a self-check error:

mypy/checker.py:3442: error: Never apply isinstance() to unexpanded types; use
mypy.types.get_proper_type() first  [misc]
            if isinstance(typ, Instance):
               ^

[jhance]

Feature is split into two error codes so that the more controversial/harder one (unused task/future) is not on by default but unused coroutine, which everyone should want, is on by default.

[JukkaL]

Looks good now! Left two small comments/suggestions. Feel free to merge after you've addressed them.

[JukkaL]

Suggested change

	"unused-coroutine", "Ensure that all coroutines are used", "Genera"
	"unused-coroutine", "Ensure that all coroutines are used", "General"

[JukkaL]

Should the description say something like "all awaitable values"?

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

edgedb (https://github.com/edgedb/edgedb)
+ edb/tools/gen_test_dumps.py:149:9: error: Value of type "Coroutine[Any, Any, None]" must be used
+ edb/tools/gen_test_dumps.py:149:9: note: Are you missing an await?

dragonchain (https://github.com/dragonchain/dragonchain)
+ dragonchain/broadcast_processor/broadcast_processor.py:190:17: error: Value of type "Coroutine[Any, Any, int]" must be used
+ dragonchain/broadcast_processor/broadcast_processor.py:190:17: note: Are you missing an await?