Yes, but in principle slower (as it cannot update its status incrementally---referred as to "end-to-end" incremental compilation and harder than the normal incr

Yes, but in principle slower (as it cannot update its status incrementally---referred as to "end-to-end" incremental compilation and harder than the normal incremental compilation producing executables) and accurate (as it leverages the compiler itself). AFAIK Racer has a heuristic engine that is faster and less accurate, and RLS falls back to Racer whenever it's appropriate.