Now that rust has async, couldn't they make fibers an opt-in replacement for threads? Your libraries use async, but you can handle those calls with fibers. Fiber support is compiled into your code but not the libraries
> Your libraries use async, but you can handle those calls with fibers. Fiber support is compiled into your code but not the libraries
We tried this! Making the attempt is actually one of the biggest reasons that we decided to remove green threads from Rust; it brought in tons of disadvantages and no real advantages.
Fibers are great because you can pretend they're threads. Everyone knows threads. Async is a legit PITA even if you're familiar with it. And the local state stored for a fake thread is often useful. When doing async I find myself frequently building hacky hashmaps to hold local variables values. websocket code is a good example of worse-case. 20k ongoing connections held by async (1 thread per core). It's a nightmare in everything except Vert.X Sync (fibers in Java) or maybe Golang and Erlang.
With fibers you get to keep all your local variables and "pretend" threads actually exist. It's a huge boon for productivity in the few languages where it's possible
(I'd also be very skeptical of the 2% figure; where did you get that from?)
Remember, "zero cost" means "zero additional cost", everything has a cost.