Add lint for missing await when returning a value inside a try / catch block

[cubuspl42]

Describe the rule you'd like to see implemented

I'd love to have a lint that detects missing await keyword after return in try / on / catch blocks of async functions.

Without the await keyword in the try block, the on / catch block is ineffective.

Examples

class _Exception {}

Future<bool> foo() async {
  throw _Exception();
}

Future<bool> bar() async {
  try {
    print("Calling foo...");
    return /* await */ foo();
  } on _Exception catch (e) {
    print("We're all good! Exception $e is handled.");
    // ...except it's not handled at all, because of the missing `await`
    return false;
  }
}

Additional context

This problem caused the customer-affecting bug in flutterfire / cloud_functions (firebase/flutterfire#2069).

It also broke my team's app yesterday.

[bwilkerson]

Does the unawaited_futures lint satisfy this need?

[cubuspl42]

@bwilkerson
Thank you for your suggestion.

Unfortunately, it seems that unawaited_futures doesn't work on return <expression>.

It would be acceptable for me that unawaited_futures was adjusted in such way, but I don't know if it would be considered a backwards-compatible change.

[stuartmorgan]

This is a lint that we would find very valuable for flutter/packages; we've had this bug a number of times across various packages, and a lint for it would be very useful.

[srawlins]

Duplicate of #58279, which has more context and discussion as well.