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.

[manfred-brands]

Note that these are bad examples as the Assert.That doesn't do any async-over-sync.
The method is called asynchronously and the result is passed to the assert.

The only place where you would want to use Assert.ThatAsync is when NUnit is calling the delegate.
This would only be the case for two asserts:

1. ThrowsConstraints where NUnit need to call the method in order to catch any exception.
2. DelayedConstraint where NUnit needs to call the method repeatedly to verify the constraint.

[manfred-brands]

Personally, I think it'd be neat if we could bundle all NUnit 4-related changes to NUnit4XXX. Thoughts?

I don't mind putting NUnit4 only diagnostics in the 4XXX range.