AsyncDestructor: replace fields with impl_did

[meithecatte]

The future and ctor fields aren't actually used, and the way they are extracted is obviously wrong – swapping the order of the items in the source code will give wrong results.

Instead, store just the LocalDefId of the impl, which is enough for the only use of this data.

[rustbot]

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[meithecatte]

#126482

@rustbot label +F-async_drop

[petrochenkov]

cc @zetanumbers

[zetanumbers]

The future and ctor fields aren't actually used, and the way they are extracted is obviously wrong – swapping the order of the items in the source code will give wrong results.

Although I should notice such an error easily, could you please provide failing code sample? Adding a test case would be the best.

[meithecatte]

There is no failing sample, because you never use these fields for anything. However:

1. Add a debug!(?future, ?ctor);.
2. Write the simplest possible AsyncDrop impl:

#![feature(async_drop)]
use std::future::{AsyncDrop, Ready};
use std::pin::Pin;

struct Foo;

impl AsyncDrop for Foo {
    type Dropper<'a> = Ready<()>;

    fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_> {
        todo!()
    }
}

3. Observe that the output of the debug! statement makes sense.
4. Swap the two associated items as follows:

impl AsyncDrop for Foo {
    fn async_drop(self: Pin<&mut Self>) -> Self::Dropper<'_> {
        todo!()
    }

    type Dropper<'a> = Ready<()>;
}

5. Observe that the output of the debug! statement is now wrong.

[zetanumbers]

Holy! Now I remember. Thanks for fixing it. LGTM @petrochenkov

[petrochenkov]

r? @petrochenkov
@bors r+ rollup

[bors]

📌 Commit a2618e1 has been approved by petrochenkov

It is now in the queue for this repository.