The problem seems to be that racer is expecting the cargo dir to be in .multirust/toolchains/<toolchain>/cargo, while in multirust-rs it's in .multirust/cargo.
DEBUG:racer::cargo: get_cargo_rootdir root is "/home/snake/.multirust/toolchains/nightly/cargo"
DEBUG:racer::cargo: get_versioned_cratefile: cargo rootdir is "/home/snake/.multirust/toolchains/nightly/cargo"
ERROR:racer::cargo: ERROR!: Error { repr: Os { code: 2, message: "No such file or directory" } } src/racer/cargo.rs 403
I'm not sure whether this deviation is an oversight or a deliberate choice. If it's the latter, then perhaps explicit support for multirust-rs vs multirust needs to be added to racer.