Concurrency is easy by default. The hard part is when you are trying to be clever.
You write concurrent code in Rust pretty much in the same way as you would write it in OpenMP, but with some extra syntax. Rust catches some mistakes automatically, but it also forces you to do some extra work. For example, you often have to wrap shared data in Arc when you convert single-threaded code to use multiple threads. And some common patterns are not easily available due to the limited ownership model. For example, you can't get mutable references to items in a shared container by thread id or loop iteration.
> For example, you can't get mutable references to items in a shared container by thread id or loop iteration.
This would be a good candidate for a specialised container that internally used unsafe. Well, thread id at least; since the user of an API doesn't provide it, you could mark the API safe, since you wouldn't have to worry about incorrect inputs.
Loop iteration would be an input to the API, so you'd mark the API unsafe.
The hard part isn't splitting loop iterations between threads, but doing so _safely_.
Proving an arbitrary loop's iterations are split in a memory safe way is an NP hard problem in C and C++, but the default behavior in Rust.