Attribute Macro pin_project_internal::pinned_drop

#[pinned_drop]

An attribute for annotating an impl block that implements Drop.

This attribute is only needed when you wish to provide a Drop impl for your type.

This impl block acts just like a normal Drop impl, except for the following two:

• drop method takes Pin<&mut Self>
• Name of the trait is PinnedDrop.

pub trait PinnedDrop {
    fn drop(self: Pin<&mut Self>);
}

#[pin_project] implements the actual Drop trait via PinnedDrop you implemented. To drop a type that implements PinnedDrop, use the drop function just like dropping a type that directly implements Drop.

In particular, it will never be called more than once, just like Drop::drop.

Example

use pin_project::{pin_project, pinned_drop};
use std::pin::Pin;

#[pin_project(PinnedDrop)]
struct Foo {
    #[pin]
    field: u8,
}

#[pinned_drop]
impl PinnedDrop for Foo {
    fn drop(self: Pin<&mut Self>) {
        println!("Dropping: {}", self.field);
    }
}

fn main() {
    let _x = Foo { field: 50 };
}

See also "pinned-drop" section of #[pin_project] attribute.

Why #[pinned_drop] attribute is needed?

Implementing PinnedDrop::drop is safe, but calling it is not safe. double dropping is unsound.

Ideally, it would be desirable to be able to forbid manual calls in the same way as Drop::drop, but the library cannot do it. So, by using macros and replacing them with private traits like the following, we prevent users from calling PinnedDrop::drop in safe code.

pub trait PinnedDrop {
    unsafe fn drop(self: Pin<&mut Self>);
}

This allows implementing Drop safely using #[pinned_drop]. Also by using the drop function just like dropping a type that directly implements Drop, can drop safely a type that implements PinnedDrop.