Trying to understand Pin and drop

help
1

Hello. I am learning about pinning and don't fully understand about drop and pinning part.

Maybe someone can show minimal example with drop and pin?

Thanks.

2

Typically, once you have pinned a struct, you can never again call a &mut self method on it, but the drop method is an exception to this rule. This makes it difficult to write no-unsafe code that uses pinning in structs, since such no-unsafe code has to add extra mitigations against this exception.

I'm not sure there's much example to show.

3

Ok. Why calling drop is dangerous in pinned struct? Struct items can change their address through &mut self? But why it dangerous if it is drop, where only deallocation happen. It called only once? Right?

4

Yes. In particular, they could be mem::swapped to another location (into the static variable, for example), and stored there, which would violate Pin contract.

5

The danger is if the drop impl moves some of the values somewhere else, making then survive the drop.

6

But i can move out some values from non pinned struct in drop too? What's the difference then?

fn inner_drop(this: Pin<&mut Type>){}

Isn't it the same as &Type, because you can't get mutable reference with Pin?

7

If it isn't pinned, you haven't promised not to move them, so it's fine in this case.

Not quite. With unsafe code you can get a mutable reference, and additionally, creating a Pin<&mut T> is a promise to never move that value ever again, which a &T would not promise.

Generally all of this is only relevant when using something like pin-project, which lets you write a struct that contains a value that should be pinned, and "forward" that pinning from your wrapper struct to the inner struct. When you use a crate like pin-project, this wrapper struct does not need any unsafe code (besides the unsafe generated by the macros in pin-project). The problems with Drop is that the pin-project macro must prevent you from generating a Drop impl for your wrapper struct, as you could otherwise write code that used pin-project, but no unsafe of your own, that could pin a struct, then move it.

When you are not using pin-project, you are using unsafe code, and then the problems are moot because by using unsafe code, you have promised to write your code correctly.

8

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