> The state-machine transformation inside rustc is a thing of beauty, but the libraries and APIs layered on top of that should have been iterated many more times before being rolled out into widespread use.

Quite. The mistake was advertising the feature as production-ready (albeit not complete), which led the ecosystem to adopt async faster than it could work out the flaws and footguns. The nice thing about the language/library split is that there is a path to good async Rust without major backwards-compatibility-breaking language changes (probably...), because the core concepts are sound, but the library ecosystem will bear the necessary brunt of evolution.