It came to my attention recently that VSCode has what it seems a well defined and comprehensive protocol for interacting with a language server:
The protocol is meant to be agnostic to platform and language (and to the IDE as well, to a degree at least). There is some preliminary interest from Eclipse committers in implementing this for Eclipse as well.
As such, it becomes obvious that it would be of interest to Rust (for racer/rustw/rls/cargo/etc) to implement their functionality under some sort of common protocol, especially as more and more language server functionality becomes available. BTW, I work not just on RustDT, but analogue IDEs for Go (GoClipse) and D (DDT), and I can definitely see the benefit of having a common protocol for complex language servers. It would be of benefit not just across IDEs, but across languages as well.