Memory pinning in Rust is not a problem that has to do with concurrency because the compiler will never relocate memory when something is referencing it. The problem is however with how stackless coroutines in general (even single-threaded ones, like generators) work. They are inherently self-referential structures, and Rust's memory model likes to pretend such structures don't exist, so you need library workarounds like `Pin` to work with them from safe code (and the discussion on whether they are actually sound is still open!)
>(and the discussion on whether they are actually sound is still open!)
Do you have a reference for this? Frankly, maybe I shouldn't ask since I still don't even understand why stackless coroutines are necessarily self-referential, but I am quite curious!
Basically the problem is that async blocks/fns/generators need to create a struct that holds all the local variables within them at any suspension/await/yield point. But local variables can contain references to other local variables, so there are parts of this struct that reference other parts of this struct. This creates two problems:
- once you create such self-references you can no longer move this struct. But moving a struct is safe, so you need some unsafe code that "promises" you this won't happen. `Pin` is a witness of such promise.
- in the memory model having an `&mut` reference to this struct means that it is the only way to access it. But this is no longer true for self referential structs, since there are other ways to access its contents, namely the fields corresponding to those local variables that reference other local variables. This is the problem that's still open.
> I still don't even understand why stackless coroutines are necessarily self-referential, but I am quite curious!
Because when stackless coroutines run they don’t have access to the stack that existed when they were created. everything that used to be on the stack needs to get packaged up in a struct (this is what `async fn` does). However now everything that used to point to something else on the stack (which rust understands and is fine with) now points to something else within the “impl Future” struct. Hence you have self referential structs.
