I was thinking: it'd be very cool to have a tool based on `rustc` that presented an annotated view of the source code, showing: - the inferred types of local variables, expressions; - hyperlinks between fn calls and the source text; - monomorphized versions of generic fns etc specific to a particular call site This might also be useful in presenting error messages and so forth. Seems like it would make a great intern project (we should have a tag for that).