New diagnostic: Consider using Assert.ThatAsync instead of Assert.That

[Bartleby2718]

Arguably, one of the biggest advantages of using NUnit 4 is: Proper async/await, you can now await the Asserts. However, it appears that NUnit.Analyzers doesn't have an analyzer or a codefix for that.

Consider the following sync-over-async tests:

[Test]
public async Task Before()
{
    Assert.That(await GetFourtyTwoAsync(), Is.EqualTo(42));
    Assert.That(await GetFourtyTwoAsync("hello world"), Is.EqualTo(42));
}

private static Task<int> GetFourtyTwoAsync() => Task.FromResult(42);

private static Task<int> GetFourtyTwoAsync(string stringToPrintBeforehand)
{
    Console.WriteLine(stringToPrintBeforehand);
    return Task.FromResult(42);
}

It'd be great if these could be fixed to:

[Test]
public async Task After()
{
    await Assert.ThatAsync(() => GetFourtyTwoAsync(), Is.EqualTo(42)); // the first argument could be simplified to just GetFourtyTwoAsync
    await Assert.ThatAsync(() => GetFourtyTwoAsync("hello world"), Is.EqualTo(42));
}

Personally, I think it'd be neat if we could bundle all NUnit 4-related changes to NUnit4XXX. Thoughts? (It does raise the question of whether we should reserve 5XXX or above as well... 🤷)

[manfred-brands]

The original test more likely looks like:

[Test]
public void Before()
{
    Assert.That(await GetFourtyTwoAsync(), Is.EqualTo(42));
    Assert.That(await GetFourtyTwoAsync("hello world"), Is.EqualTo(42));
}

So any CodeFix would also possibly have to change the method declaration.