What is a Pin

[minor hornet]

I can't understand what is a Pin and why is it needed, if someone could explain ?

https://doc.rust-lang.org/beta/unstable-book/language-features/generators.html

The question comes from this page, where we have to pin the Generator to use the resume() function, why is it needed ?

[minor hornet]

I'm reading that

Pin works in tandem with the Unpin marker. Pinning makes it possible to guarantee that an object implementing !Unpin won't ever be moved

But why would an object move? 🤔

Someone has an example ?

[prime stratus]

https://doc.rust-lang.org/std/pin/

The explanation there is pretty good ^

[hoary plover]

An object would move because you do this:

let new_place = object;

This is not actually what pin is for, though.

[minor hornet]

I'm reading, thanks for the link I was on https://rust-lang.github.io/async-book/04_pinning/01_chapter.html

[hoary plover]

The idea is as follows:

• there are cases, usually around self-referential types, where it would be really bad if a value moved around in memory.
• as long as all code that manipulates these values is unsafe, this is fine, it can just be careful and manipulate them by pointer.
• but we want to let safe code manipulate these values too.
• so, we need a way to prevent safe code holding a reference (or another kind of pointer) to one from Doing Bad Things
• thus, we created the notion of pinning

[minor hornet]

Alright, I've read a lot of Pinning regarding Future and now Generator

It is the same use case ?

[hoary plover]

Yes. Futures need this because they're implemented as generators, more or less.

You'll probably find more information by looking into futures though

[minor hornet]

I mean, do we want to pin a Future because, we don't want it to move, or we don't want the inner values to not be moved ?

[hoary plover]

Particularly, into the desugaring of async fn

[hoary plover]

Those are the same thing.

[minor hornet]

I'll look into it

[hoary plover]

The inner values of a future are usually externally inaccessible, so they'd move along with the whole

[minor hornet]

Alright, like a struct ?

Thanks both of you @prime stratus @hoary plover 🙏

[hoary plover]

The desugaring actually uses an enum, but yes, it's the same idea.

[minor hornet]

Alright !

I'll check

How do I close the chann ?

Just closed or mark as resolved somewhere ?