dotnet/runtime#85612 plans to centralize the locations of attributes defined by the C# compiler into corelib as public types to reduce duplicate definitions. As part of that, we'll need to react and validate scenarios:

• Does the issue have the full list of attributes currently synthesized by Roslyn? (cataloged in Attribute definitions included in assemblies using Nullable on the latest framework runtime#85612 (comment))
• What order do we lookup existing attribute definitions? Likely we need to prioritize attributes coming from corlib, meaning coming from the assembly that defines System.Object and has no references.
• What is the compiler's behavior when it sees these attributes via an assembly reference it has IVT to today? If that assembly is recompiled with these new definitions and grants IVTs, does it need to include type forward attributes to keep things working?

/cc @jaredpar for anything I'm missing in this list.