Agreed, I hope that a good "Language service" API is on the horizon, so that tool authors don't need to reinvent the wheel in order to support parsing, error display, highlighting, completions and so on.

A step in the right direction is the possibility of getting machine-readable (json) error messages from the compiler.

Being able to just call the compiler as a library to produce AST's from source would be very handy, though it would probably complicate the compiler to be able to provide useful output from broken/incomplete source, which is critical for tool use.