Rust used to have a light runtime that managed asynchronous operations using green threads. It was eventually discarded because it caused an unavoidable perform

Rust used to have a light runtime that managed asynchronous operations using green threads. It was eventually discarded because it caused an unavoidable performance hit even when not using it (because the low-level runtime and I/O methods all needed virtual dispatch) and it wasn't providing any clear benefit over just using OS threads.