[unawaited_futures] Special-case future.then((_) => completer.complete[Error](...))

[ochafik]

This is one of the obvious cases that don't need to be awaited, which I originally intended to add to https://github.com/dart-lang/linter/blob/master/lib/src/rules/unawaited_futures.dart#L83 but was too lazy.

Turns out this is a frequent use-case for the new unawaited helper from https://github.com/dart-lang/pedantic, so would justify the extra work IMO.

@davidmorgan WDYT?

[davidmorgan]

I don't feel strongly about it. In the one example I've seen it's actually nice to have use of futures inside an async block highlighted with 'unawaited'; it feels like this should be quite rare. But I don't have a lot of evidence for that...

[rkj]

Given this advice:
https://www.dartlang.org/guides/language/effective-dart/usage#prefer-asyncawait-over-using-raw-futures
I feel like the then syntax should be changed to await, or async removed from the method annotation.

[matanlurey]

Using future.then in an async { ... } function just seems wrong.

If it is intentional, unawaited(...) seems entirely appropriate and worth the boiler-plate.

[srawlins]

Future.whenComplete is another case. #57400