This explains fairly clearly a bit of rust that, while not the most obscure, is IMO fairly poorly explained in most resources I’ve found, and covers a lot of edges that are really useful to understand… I can keep digging and digging into the details presented. I find it absurd to say it’s like a freshman in college - I’d love to hire those freshman if you can point them out:
> Explain when pin is important in rust when dealing with asynchronous code, particularly when dealing with Box<dyn> types.
In Rust, Pin is a concept that is central to understanding the implementation of asynchronous operations. To explain why, let's first cover the basics.
Asynchronous tasks often involve future types that encapsulate a computation that will produce a value at some point in the future. These futures may need to poll other futures, which in turn might not be ready, causing them to be suspended and resumed later on. This suspension and resumption is where the Pin concept comes in.
By default, Rust allows moving types around in memory. However, when you have self-referential structures, like many of the future types in asynchronous programming, this can lead to problems. A future may hold a reference to another part of itself, and if the future is moved in memory, that internal reference may become invalid.
Pin is Rust's solution to this problem. If a type is "pinned" (wrapped in a Pin), it is a guarantee that the memory location of the value inside will not change. This means that you can safely keep references to different parts of the same structure because you know they won't move around.
Box<dyn Future> is a heap-allocated future object. Using Pin with Box<dyn Future> (i.e., Pin<Box<dyn Future>>) is common in async code because it guarantees that the future object will not be moved around in memory, making it safe to poll.
However, one should note that simply creating a Pin<Box<T>> does not mean that the T will stay put; it's also necessary to ensure that T does not move itself or its interior around. The contract of Pin is such that, once a value has been pinned, it is not allowed to move or have its memory invalidated until it is dropped. For a type to uphold these guarantees, it must be Unpin or ensure that all its operations maintain these guarantees. Most native Rust types are Unpin and therefore don't need to worry about these guarantees.
In short, Pin is important in Rust's async code as it provides a mechanism to safely handle self-referential structures, which are common in async programming. The Rust async ecosystem uses Pin to ensure that once a future begins executing (is polled), it will remain at a fixed location in memory until it has completed.