Okay so if you want racer to work with multirust, you apparently need the source of Rust installed too, and multirust doesn't use that.

So you have to clone rust source, then set a path variable to the rust source, then reinstall.

Then if you reinstall the extension, Visual Code can use the addon to do racer and code checking for you.

...so this is what an attention trap is like on adderall. That was two and half hours gone. Oops.