I believe the answer is "that implies a runtime", and Rust as a whole is not willing to pull that up into a language requirement. This is in contrast to Haskel

I believe the answer is "that implies a runtime", and Rust as a whole is not willing to pull that up into a language requirement. This is in contrast to Haskell, Go, dynamic scripting languages, and, frankly, nearly every other language on the market. Almost everything has a runtime nowadays, and while each individually may be fine they don't always play well together. It is important that as C rides into the sunset (optimistic and aspirational, sure, but I hope and believe also true) and C++ becomes an ever more complex choice to make for various reasons that we have a high-power very systems-oriented programming language that will make that choice, because someone needs to.