Suppress nullability warnings after Is.Not.Null constraint

[Dreamescaper]

As discussed here nunit/nunit#3376 (comment), it would be great to suppress nullability warnings after Is.Not.Null asserts, e.g.

string? str = GetNullable();
Assert.That(str, Is.Not.Null);
Assert.That(str.Length, Is.Not.Zero);  // <----- should be no warning here

Unfortunately, we cannot impact roslyn's nullability analysis, only suppress analyzers (see dotnet/roslyn#32042 (comment)), which means that at least very basic logic should be re-invented.

Cases that I think should be covered:
Assert.That(obj?.PropA?.PropB, Is.Not.Null)
Assert.That(obj, Is.Not.Null & Is.Not.Empty)
Assert.That(obj, Is.Not.Null.And.Not.Empty)

Assert.Multiple(() =>
{
	string? str = GetNullable();
	Assert.That(str, Is.Not.Null);
	Assert.That(str.Length, Is.Not.Zero);  // <----- warning should remain here
});

[mikkelbu]

I can definitely see the benefit of this, but also the complexity if we want to handle more than the most basic cases, as we would have to implement the same flow analysis as in the compiler.

So the easiest is probably to start with the most basic example and then build upon this.

[Dreamescaper]

The only concern is - doing that would probably require to drop VS2017 support (or provide separate package for VS2017).
I don't know any way to reference Roslyn 3.0 package and maintain compatibility with VS2017.

[manfred-brands]

I'm looking into the use of a DiagnosticsSuppressor.