Messages in this thread | | | Date | Thu, 25 May 2023 17:32:17 +0100 | From | Gary Guo <> | 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
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