Linter to suggest removing the `Future` return type for the synchronous option

[FMorschel]

After some discussions over at #58862 (comment), if we ever end up having some form of lint that was merely a hint or nudge to point to the user another option, I'd like to ask for a feature like #59814 but that would encourage users to change the return type for the function.

As per @scheglov in #59814 (comment):

When we can change the return type, it would be really good to do so.

Benchmark code

Future<int> getValueAsync() async {
  return 42;
}

int getValueSync() {
  return 42;
}

Future<void> runAsyncBenchmark() async {
  final timer = Stopwatch()..start();
  var acc = 0;
  for (var i = 0; i < 10000000; i++) {
    acc = (acc + await getValueAsync()) & 0xFFFFFF;
  }
  timer.stop();
  print('Async Benchmark: ${timer.elapsedMicroseconds} us');
}

Future<void> runSyncBenchmark() async {
  final timer = Stopwatch()..start();
  var acc = 0;
  for (var i = 0; i < 10000000; i++) {
    acc = (acc + getValueSync()) & 0xFFFFFF;
  }
  timer.stop();
  print(' Sync Benchmark: ${timer.elapsedMicroseconds} us');
}

void main() async {
  // Warm-up (very basic in this example)
  print('--- warm-up');
  await runAsyncBenchmark();
  await runSyncBenchmark();

  // Run benchmarks for measurement
  print('--- benchmark');
  await runAsyncBenchmark();
  await runSyncBenchmark();
}

Results

--- warm-up
Async Benchmark: 895403 us
 Sync Benchmark: 3889 us
--- benchmark
Async Benchmark: 888209 us
 Sync Benchmark: 3973 us

It looks more than 100x faster for this trivial code.

Opening this issue so this idea is not lost and others can chime in.