[shellac]

Sibling comments have explained the detail. The pithier explanation perhaps is that rust is intended as a 'systems language', and it interprets that as meaning there should be no runtime. Or, more simply, it ought to be possible to call a rust function from C without providing an additional argument that encapsulates rust 'environment'. (C effectively defining this area)

Go and Java (with Loom) have these lovely facilities, but it is hard to interface with them if your language lacks these features. I find it odd that C#, javascript, and python don't provide the smoother async Go experience despite having runtimes / VMs.