Regarding thread::scoped, I think that kind of issue isn't unique to Rust (Java, a language with a simpler memory model, has had all kinds of weird corner case bugs, especially around memory and concurrency). It's also, again, more an issue of "safe manual memory management is hard": the simple ways to avoid that issue would have been "GC everything", "sacrifice memory safety", or "remove reference counted smart pointers", which contradict Rust's goals.