[Proposal] Extend MemberNotNullAttribute to support ReturnValue as a target

[hwoodiwiss]

I've come across cases a number of times where I have a method that guarantees that a child property on a type that it returns will be a non-null value, but to my knowledge there is no way at the moment to annotate the method to be able to indicate this for nullability static analysis.

For example:

#nullable enable

public class ChildClass { public string SomeString { get; set; } = string.Empty; }
public class ParentClass { public ChildClass? Child { get; set; } }

public static class ParentFactory
{
    public static ParentClass CreateParent()
    {
        return new()
        {
            Child = new(),
        };
    }

    public static void UseParent()
    {
        var child = CreateParent().Child.SomeString; // <--- Warning CS8602 - Child may be null here
    }
}

It would be useful to be able to annotate CreateParent with [return: MemberNotNull(nameof(ParentClass.Child))] and have that information be captured in the nullability analysis to avoid that warning.