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rust: sync: document `PhantomData` in `Arc` 

Add a comment explaining the relevant semantics of `PhantomData`. This
should help future readers who may, as I did, assume that this field is
redundant at first glance.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241107-simplify-arc-v2-1-7256e638aac1@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Tamir Duberstein 2024-11-07 05:36:46 -05:00 committed by Miguel Ojeda
parent 3f4223c007
commit 2dde1c8b04
1 changed files with 8 additions and 0 deletions
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8
rust/kernel/sync/arc.rs
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@ -127,6 +127,14 @@ mod std_vendor;
/// ```
pub struct Arc<T: ?Sized> {
ptr: NonNull<ArcInner<T>>,
// NB: this informs dropck that objects of type `ArcInner<T>` may be used in `<Arc<T> as
// Drop>::drop`. Note that dropck already assumes that objects of type `T` may be used in
// `<Arc<T> as Drop>::drop` and the distinction between `T` and `ArcInner<T>` is not presently
// meaningful with respect to dropck - but this may change in the future so this is left here
// out of an abundance of caution.
//
// See https://doc.rust-lang.org/nomicon/phantom-data.html#generic-parameters-and-drop-checking
// for more detail on the semantics of dropck in the presence of `PhantomData`.
_p: PhantomData<ArcInner<T>>,
}