[JoshTriplett]

I'm not sure how meaningful that would be. Selectable runtimes make a huge amount of sense for interpreted languages or languages with a runtime. Rust is compiled to a binary. There are libraries that make it easy to set up a Lambda runtime, and guides for deploying that runtime; beyond that, a Lambda runtime is "run this program, leave it running, answer requests", and I don't know how standardized that could easily become with a compiled language that doesn't need a runtime or interpreter.