Rick said in a comment,

You said: “Philosophically, it would be nice to have some way to leverage the knowledge that the compiler has about the code, but we don't currently have any plans in that area.“

I've often thought that its pretty difficult for tools to do meaningfull things with most source code due to the complexities of parsing. Any chance we'll see the front-end and back-end (and maybe even each phase) of C# compiler be accessible independantly?

I'd like to clarify a bit what I said.

We don't currently have any plans in this area, but we certainly do recognize the utility of providing this sort of access for customers, and would like to be able to do it in the future. And, to answer one of Rick's other questions, Intellisense and refactoring do share the compiler code, but the interface between them is complex and not terribly pretty, and therefore not something we'd like to expose.