I use at least 3 separate runtimes: tokio and 2 no_std runtimes (rtic and embassy). The latter would probably not be possible at all if there was an "official" runtime, because the official runtime would inevitably require allocation, and if it existed they wouldn't bother writing async in a flexible enough way that you could use it without an allocator.
The way async is implemented in rust is actually technically quite impressive, and would almost certainly not exist if there were some official green thread solution.
You could solve async/non-async polymorphism via the introduction of HKTs (and monads) - perhaps eventually they will be forced to do that.
In the mean time, if they can make a few changes like stabilizing TAITs and async traits, that would go a long way to improving the ergos of async.
The way async is implemented in rust is actually technically quite impressive, and would almost certainly not exist if there were some official green thread solution.
You could solve async/non-async polymorphism via the introduction of HKTs (and monads) - perhaps eventually they will be forced to do that.
In the mean time, if they can make a few changes like stabilizing TAITs and async traits, that would go a long way to improving the ergos of async.