Re: async in traits, the feature was delayed because it relied on the "Generic Associated Types" and "Impl Trait in Traits" features. If Rust delayed the whole `async` feature for working on those pretty type-theoretic features what would you have thought?

> requires full access to source code

What do you mean? Editions don't require full access to source code. Rust in general relies heavily on having access to source code, but that has nothing to do with how editions work

I don't fully agree with this. Yes, surely shared mutable state can suffer from similar issues, however the cooperative nature of coroutines makes this much easier to handle. OS threads are preemptive and actually run in parallel, so you have to be aware of CPU concurrency and always be ready for a context switch.