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Date:   Wed, 24 May 2023 11:11:27 +0000
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Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}`
From:   Alice Ryhl <aliceryhl@google.com>
To:     nmi@metaspace.dk
Cc:     alex.gaynor@gmail.com, aliceryhl@google.com,
        benno.lossin@proton.me, bjorn3_gh@protonmail.com,
        boqun.feng@gmail.com, gary@garyguo.net, jiangshanlai@gmail.com,
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Andreas Hindborg <nmi@metaspace.dk> writes:
> Alice Ryhl <aliceryhl@google.com> writes:
>> +        // This preserves the metadata in the pointer, if any.
>> +        let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
> 
> I can't follow this. How does this work? `ptr` was for field
> `inner.data: T`, but we are casting to `ArcInner<T>`.
> 
>> +        let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
>> +        let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> 
> Metadata was obtained from a pointer pointing to `inner.data`, we then
> move it back to beginning of `ArcInner<T>` and then reconstruct the
> potentially fat pointer with metadata from the pointer to `T`? How can
> this be right?

The metadata of a struct is always the metadata of its last field, so
both `*mut T` and `*mut ArcInner<T>` have the same metadata. Because of
that, moving the metadata over from one type to the other is ok.

The reason that I cast to an `ArcInner<T>` pointer before calling
`metadata` is because I get a type mismatch otherwise for the metadata,
since the compiler doesn't unify the metadata types when the type is
generic.

Alice