That would mean a runtime in the standard library, which was explicitly rejected. Different runtimes have different pros and cons and different use cases, choos

That would mean a runtime in the standard library, which was explicitly rejected. Different runtimes have different pros and cons and different use cases, choosing one and putting it in the standard library means that all others are second class and probably not well supported. For example, some might want an io-uring based async runtime, while others want a runtime-per-core system for a web server, and another might want a simple lightweight runtime for an embedded environment. Someone else wants to use raw OS primitives without ever spawning a runtime (to use block_on and an async function that does IO, you still need a runtime running in the background). A better solution is to provide standard interfaces for runtimes and libraries to develop against, something that is being worked on.