To be clear, in this comment, I am referring to async in the broadest sense. I am specifically NOT referring to the Rust compiler keyword and runtimes (like tokio) that work with rust future state machines.

If I understand OP correctly his current proposal is to create a different runtime for rust. That runtime happens to be written in scheme using his own compiler. It is meant to make writing async rust applications easier by having simple rust interop.

Given the authors implicit argument that an async runtime with a stack-based context switching model can provide better debugging. Is it possible to write something like that in rust? That may be better for some use cases, as it wouldn't result in a multi-language project.

Unlikely. C and Rust have run-time environments. Just not as elaborate as the JVM for Java or a web browser for JS and WASM.

It could only be about adding a second run-time environment to the same operating system process. That’s what the questions seems to be enquiring about.

And it’s about instrumenting the Rust run-time environment with a REPL and human-friendly run-time data structures.