Comment by alkonaut

I assume they aren't, because their Resharper codebase already had the bits needed. Still, if you start writing a C# IDE from scratch, wouldn't the obvious choice be using Roslyn for everything from autocomplete to refactorings? Easily half to 90% of the work going into an IDE has to be in the language services (if you want to do the really tricky things well, like completing broken syntax).