Messages in this thread | | | Date | Wed, 24 May 2023 11:11:27 +0000 | Subject | Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` | From | Alice Ryhl <> |
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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
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