• Add support for #nullable (enable | disable | restore) [ warnings | annotations ])
• Track a "nullable warning context"
• Add support for /nullable:annotations
• Add support for /NoWarn:nullable

https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-13.md

1. There is a nullable warning context as a separate "bit". It can be set by default at project-level, and overridden with #nullable directives. It needs to be on to get nullable warnings at a given point in the source.
2. Individual warnings continue to be able to be individually disabled at the project-level with /NoWarn and disabled or restored in source with #pragma warning ....
3. Additionally there is a "virtual warning identifier", nullable that can be used in /NoWarn at the project level, and that individually disables all warnings related to the nullable feature.