proposal: lint to catch unawaited futures in try blocks

[eseidel]

The gotcha occurs when you return Future<T> from Future<T> foo async {} instead of T and that return is inside a try block.

https://twitter.com/_eseidel/status/1660092263190650881

https://dartpad.dev/?id=f4c93eb9f9e24d43079cbc859a867894

class ApiException implements Exception {
  ApiException();
}

Future<void> inner() async {
  throw ApiException();
}

Future<void> _middle() async {
  try {
    return inner();
  } on ApiException catch (e) {
    print('inner: $e');
  }
}

Future<void> outer() async {
  try {
    await _middle();
  } on ApiException catch (e) {
    print('outer: $e');
  }
}

void main() async {
  await outer();
}

Would love a lint that would have caught that (and saved me -- and clearly others according to responses on that twitter thread) a while of searching for the issue.

Discussion checklist

• List any existing rules this proposal modifies, complements, overlaps or conflicts with.

unawaited_futures could have some overlap, although it doesn't trigger here.

unnecessary_await_in_return seems in direct conflict.

• List any relevant issues (reported here, the [SDK Tracker], or elsewhere).

• If there's any prior art (e.g., in other linters), please add references here.
  https://dcm.dev/docs/teams/rules/common/prefer-return-await/

• If this proposal corresponds to [Effective Dart] or [Flutter Style Guide] advice, please call it out. (If there isn't any corresponding advice, should there be?)

• If this proposal is motivated by real-world examples, please provide as many details as you can. Demonstrating potential impact is especially valuable.

Arguably the real bug is that Future<T> foo async {} allows returning Future<T> directly at all, since it nominally expects you to return a T. But presumably that's an intentional language decision.

[eernstg]

Perhaps this issue reports the same behavior as #44395, where it is reported as a bug?

[eseidel]

Also probably a dupe of #58279.

I'm not trying to pick a fight. :) Just trying to get to some outcome for users (like me) so I don't spend time chasing exceptions coming from inside code I was expecting was catching them. 🙃

[eseidel]

Looks like lots of bugs covering this. Closing this one. Thank you!

[incendial]

@eseidel hey, considering you added a link to the DCM's rule, would you like to try it? Or is there anything that stops you?