>You can't build and application with the specialized thread-per core or single-threaded runtime thing if you wanted to because of pervasive incidental complexity. [...] It's frustrating and I go out of my way to avoid Tokio because of it.

At $dayjob we have built a large codebase (high-throughput message broker) using the thread-per-core model with tokio (ie one worker thread per CPU, pinned to that CPU, driving a single-threaded tokio Runtime) and have not had any problems. Much of our async code is !Send or !Sync (Rc, RefCell, etc) precisely because we want it to benefit from not needing to run under the default tokio multi-threaded runtime.

We don't use many external libs for async though, which is what seems to be the source of your problems. Mostly just tokio and futures-* crates.

I might be misremembering and the overbearing constraints might be in Axum (which is still a Tokio project). External libs are a huge problem in this area in general, yeah.