jasonbock, 
Just saw a very weird issue with C# source generators in Rider.
- Reference a NuGet package that does source generation
- Write code that will trigger the generation of source.
What happens is that Rider doesn't "see" the new generated type, BUT, a build works, and the test runs as well.
The only way my co-worker got around this was to use EmitCompilerGeneratedFiles. (I don't use Rider, I use VS, and everything works fine there).
Tagging @khalidabuhakmeh for visibility.