Install the package
language-rust in Atom (Preferences->Packages) or Atom's package manager from a shell:
$ apm install language-rust
Grammar rules were written from scratch. They seem to work for most cases. However there are most certainly still a few cases that result in wrong syntax highlighting. Feel free to fix it and send a pull request, or open an issue to report it.
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.