linux-kernel - Re: [PATCH] rust: arc: remove unused PhantomData

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Message-ID: <CAH5fLgjXXE32k2VuC9yGrNG7ib5vo7V+fsvRxWG0ijopK7MDCQ@mail.gmail.com>
Date: Wed, 6 Nov 2024 10:26:26 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: arc: remove unused PhantomData

On Tue, Nov 5, 2024 at 9:13 PM Tamir Duberstein <tamird@...il.com> wrote:
>
> On Tue, Nov 5, 2024 at 8:29 AM Miguel Ojeda
> <miguel.ojeda.sandonis@...il.com> wrote:
> >
> > On Mon, Nov 4, 2024 at 11:42 PM Boqun Feng <boqun.feng@...il.com> wrote:
> > >
> > > I believe we need this `PhantomData` to inform drop chec [1] that the
> > > drop of `Arc` may result into the drop of an `ArcInner<T>`. Rust std
> > > `Arc` has the similar definition [2], you can find more information
> > > about PhantomData usage on drop checking at [3].
> >
> > Hmm... But they use `may_dangle` in their `Drop` and we don't (and we
> > have a `Drop` unlike something like `Unique`), no? Or am I confused?
> > i.e. if I understand correctly, that would have been needed in the
> > past, but not anymore.
>
> Doing a bit of archaeology I found the reasoning for the presence of
> `PhantomData` in std's `Arc`[0]. The TL;DR is that the presence of a
> type parameter `T` implies "owns T", but `Arc` owns `ArcInner<T>`
> rather than `T`. Thus in order to get correct dropck behavior it is
> necessary to opt out of "owns T" using `may_dangle` and opt into "owns
> ArcInner<T>" using `PhantomData`.
>
> Please check my understanding; I couldn't find detailed documentation
> of the interaction between `may_dangle` and `PhantomData`. If this
> sounds correct, should we add `may_dangle` to our dropck? compile-fail
> tests would be useful here.

We don't *have* to use #[may_dangle]. Using it may allow more stuff,
but it's not a problem for it to be missing. We probably don't want to
use it since it's unstable.

Since we don't use #[may_dangle], we don't *need* the PhantomData
field. Having it doesn't change anything.

Alice