Gosh, I think that means even when your code is wholly running on workers (where you would be able to use the atomic wait mentioned in the comment), it still will busy loop, doesn't it? At least it's within the allocator and not in the general implementation of Mutex... I think?

Yes that first sentence is correct and that's an unfortunate side effect. The Rust ecosystem eventually needs to evolve its multithreaded Wasm approach. Atomics are still only supported on Nightly Rust but it's been that way for 7+ years now.

And yes you're right again it's only in the allocator.