Messages in this thread | | | From | Andreas Hindborg <> | Subject | Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` | Date | Thu, 25 May 2023 09:45:29 +0200 |
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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
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