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Date:   Thu, 25 May 2023 17:32:17 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Andreas Hindborg <nmi@metaspace.dk>
Cc:     Alice Ryhl <aliceryhl@google.com>, alex.gaynor@gmail.com,
        benno.lossin@proton.me, bjorn3_gh@protonmail.com,
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        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}`
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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

> For structs whose last field is a DST, metadata is the metadata for the last field

Best,
Gary