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From:   Andreas Hindborg <nmi@metaspace.dk>
To:     Gary Guo <gary@garyguo.net>
Cc:     Alice Ryhl <aliceryhl@google.com>, alex.gaynor@gmail.com,
        benno.lossin@proton.me, bjorn3_gh@protonmail.com,
        boqun.feng@gmail.com, jiangshanlai@gmail.com,
        linux-kernel@vger.kernel.org, ojeda@kernel.org,
        patches@lists.linux.dev, rust-for-linux@vger.kernel.org,
        tj@kernel.org, wedsonaf@gmail.com
Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}`
Date:   Tue, 30 May 2023 09:23:29 +0200
In-reply-to: <20230525173217.6eccda7b.gary@garyguo.net>
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Gary Guo <gary@garyguo.net> writes:

> On Thu, 25 May 2023 09:45:29 +0200
> Andreas Hindborg <nmi@metaspace.dk> wrote:
>
>> Alice Ryhl <aliceryhl@google.com> writes:
>> 
>> > 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.  
>> 
>> OK, cool. In that case, since this is common knowledge (is it?),
>> could you maybe include a link to the relevant documentation, or a
>> comment indicating why this is OK?
>> 
>> BR Andreas
>
> This is documented in the doc of Pointee trait:
>
> https://doc.rust-lang.org/std/ptr/trait.Pointee.html

Nice. I think I forgot a _not_ in my last message. I think it would be
nice to add a comment with a link to this documentation and perhaps a
note as to why this works.

BR Andreas