The post concludes by saying it's hopeless to get this stuff implemented because back compat, but I do think that that is true. (It might be hopeless for other reasons. It certainly felt hopeless in 2015.)

All this is about adding things to the language. That's backwards compatible. E.g. Drop doesn't need to be changed, because from every Drop instance a DropVer2 instance can be written instead. async v1 can also continue to exist, just by continuing to generate it's existing shitty unsafe code. And if someone wants something better, they can just use async v2 instead.

People get all freaked out about changing languages, but IMO the FUD is entirely due to sloppy imperative monkey brain. Languages are ideas, and ideas are immutable. The actual question is always, can we do "safe FFI" between two languages. Safe FFI between Rust Edition 20WX and 20YZ is so trivial that people forget to think about it that way. C and C++ is better since C "continues to exist", but of course the bar for "safe FFI" is so low when the language themselves are unsafe within themselves so that safety between them couldn't mean very much.

With harder edition breaks like this, the "safe FFI" mentality actually yields fruit.