> rust async seems to be a terrible design blunder [...] that could destroy the community and render the language worthless

What an extreme take!

Async Rust as-it-is makes perfect sense for Rust's primary audience: folks who desire control/performance enough that they would otherwise be using C/C++. To call async an "efficiency hack" is to underestimate how important that efficiency is to Rust's existence. If async didn't work the way it does, it wouldn't have gained traction within the Rust community (folks would have just ignored it and kept writing state machines by hand) and it wouldn't have been able to act as a draw for C/C++ developers. It's a killer feature, and has enabled the language to thrive.

Not only is it a killer feature, the way it is designed is the only way it could have been. See: https://without.boats/blog/why-async-rust/