Has anyone tried modifying rustc to use ra_syntax to parse code instead of the existing parser?
That's the plan, but I don't know if matklad is working on it at this point. It needs to be coordinated with the larger Rust project, so it's probably going to need at least an RFC which I'm not sure is written yet.
They called it "parser library-ification", you could search it in Zulip for more details.
Not yet. I would estimate the work to get "something" in terms of unified parser as a week for someone who is familiar with both rustc and rust-analyzer. The work to do it properly... three months?