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Date:   Wed, 04 Jan 2023 17:13:36 +0100
Message-Id: <CPJJEXA0W9QT.K7XCYZGX0NU3@vincent>
Cc:     "Miguel Ojeda" <ojeda@kernel.org>,
        "Alex Gaynor" <alex.gaynor@gmail.com>,
        "Boqun Feng" <boqun.feng@gmail.com>, "Gary Guo" <gary@garyguo.net>,
        =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/7] rust: sync: allow coercion from `Arc<T>` to
 `Arc<U>`
From:   "Vincenzo" <vincenzopalazzodev@gmail.com>
To:     "Wedson Almeida Filho" <wedsonaf@gmail.com>,
        <rust-for-linux@vger.kernel.org>
References: <20221228060346.352362-1-wedsonaf@gmail.com>
 <20221228060346.352362-3-wedsonaf@gmail.com>
In-Reply-To: <20221228060346.352362-3-wedsonaf@gmail.com>
Precedence: bulk


Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

On Wed Dec 28, 2022 at 7:03 AM CET, Wedson Almeida Filho wrote:
> The coercion is only allowed if `U` is a compatible dynamically-sized
> type (DST). For example, if we have some type `X` that implements trait
> `Y`, then this allows `Arc<X>` to be coerced into `Arc<dyn Y>`.
>
> Suggested-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> ---
>  rust/kernel/lib.rs      |  2 ++
>  rust/kernel/sync/arc.rs | 27 ++++++++++++++++++++++++++-
>  2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 1a10f7c0ddd9..4bde65e7b06b 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -13,8 +13,10 @@
> =20
>  #![no_std]
>  #![feature(allocator_api)]
> +#![feature(coerce_unsized)]
>  #![feature(core_ffi_c)]
>  #![feature(receiver_trait)]
> +#![feature(unsize)]
> =20
>  // Ensure conditional compilation based on the kernel configuration work=
s;
>  // otherwise we may silently break things like initcall handling.
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index e2eb0e67d483..dbc7596cc3ce 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -17,7 +17,11 @@
> =20
>  use crate::{bindings, error::Result, types::Opaque};
>  use alloc::boxed::Box;
> -use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
> +use core::{
> +    marker::{PhantomData, Unsize},
> +    ops::Deref,
> +    ptr::NonNull,
> +};
> =20
>  /// A reference-counted pointer to an instance of `T`.
>  ///
> @@ -82,6 +86,23 @@ use core::{marker::PhantomData, ops::Deref, ptr::NonNu=
ll};
>  /// obj.use_reference();
>  /// obj.take_over();
>  /// ```
> +///
> +/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
> +///
> +/// ```
> +/// use kernel::sync::Arc;
> +///
> +/// trait MyTrait {}
> +///
> +/// struct Example;
> +/// impl MyTrait for Example {}
> +///
> +/// // `obj` has type `Arc<Example>`.
> +/// let obj: Arc<Example> =3D Arc::try_new(Example)?;
> +///
> +/// // `coerced` has type `Arc<dyn MyTrait>`.
> +/// let coerced: Arc<dyn MyTrait> =3D obj;
> +/// ```
>  pub struct Arc<T: ?Sized> {
>      ptr: NonNull<ArcInner<T>>,
>      _p: PhantomData<ArcInner<T>>,
> @@ -96,6 +117,10 @@ struct ArcInner<T: ?Sized> {
>  // This is to allow [`Arc`] (and variants) to be used as the type of `se=
lf`.
>  impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
> =20
> +// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be con=
verted to the
> +// dynamically-sized type (DST) `U`.
> +impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> =
for Arc<T> {}
> +
>  // SAFETY: It is safe to send `Arc<T>` to another thread when the underl=
ying `T` is `Sync` because
>  // it effectively means sharing `&T` (which is safe because `T` is `Sync=
`); additionally, it needs
>  // `T` to be `Send` because any thread that has an `Arc<T>` may ultimate=
ly access `T` directly, for
> --=20
> 2.34.1