Didn't realize Rust had support for this. At the end of your recent example, it pretty much says doing so sacrifices the runtime entirely. I know Go doesn't support calling from C for reasons similar to this. Where does further development on this functionality sit on the priorities list for Rust?
Indeed, being able to forgo the runtime entirely is what makes it possible to rewrite the Rust runtime in Rust (as per the OP). Wouldn't be very useful to have a runtime that itself required a separate runtime. :)
As for "further development", what would you like to see? Currently the most prominent capabilities of the runtime are the lightweight task system and garbage collection. It might be theoretically possible to allow uses of the task system to gracefully degrade to using system threads, and likewise it might be possible for uses of the GC to degrade to refcounting (without cycle collection).
Being able to run runtime-less and still use the language is quite high on the priorities, and I believe moving GC to pluggable library types and cleaning up the standard library feed into that.
You don't actually have to sacrifice the runtime entirely anymore; since the runtime is written in Rust you can just start it manually if you want. Making this smoother is a priority. (Brian mentions this in the email, regarding #[start].)
