Today it is not possible to do this. In the eventual future it should be possible to run external build tools, but Rust is a complex one to express. I do not believe there is any roadmap to do this today.
If you're interested in doing something like this today, and you're willing to put a lot of work into it, you can go down the path of using bazel which supports rust and Swift and they could theoretically intermingle in the same build that way (although I don't know of anyone doing this today for rust specifically)