> but I genuinely can't understand how anybody could look at the mess that's Rust's async and think that it was a good design for a language that already had the reputation of being very complicated to write.
Rust adopted the stackless coroutine model for async tasks based on its constraints, such as having a minimal runtime by default, not requiring heap allocations left and right, and being amenable to aggressive optimizations such as inlining. The function coloring problem ("contamination") is an unfortunate consequence. The Rust devs are currently working on an effects system to fix this. Missing features such as standard async traits, async functions in traits, and executor-agnosticism are also valid complaints. Considering Rust's strict backwards compatibility guarantee, some of these will take a long time.
I like to think of Rust's "async story" as a good analogue to Rust's "story" in general. The Rust devs work hard to deliver backwards compatible, efficient, performant features at the cost of programmer comfort (ballooning complexity, edge cases that don't compile, etc.) and compile time, mainly. Of course, they try to resolve the regressions too, but there's only so much that can be done after the fact. Those are just the tradeoffs the Rust language embodies, and at this point I don't expect anything more or less. I like Rust too, but there are many reasons others may not. The still-developing ecosystem is a prominent one.