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References: <20230224-rust-arc-v1-0-568eea613a41@asahilina.net>
 <20230224-rust-arc-v1-1-568eea613a41@asahilina.net>
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From:   Andreas Hindborg <nmi@metaspace.dk>
To:     Asahi Lina <lina@asahilina.net>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@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>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        asahi@lists.linux.dev
Subject: Re: [PATCH 1/2] rust: sync: arc: implement Arc<dyn Any + Send +
 Sync>::downcast()
Date:   Mon, 27 Feb 2023 12:46:45 +0100
In-reply-to: <20230224-rust-arc-v1-1-568eea613a41@asahilina.net>
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Asahi Lina <lina@asahilina.net> writes:

> This mirrors the standard library's alloc::sync::Arc::downcast().
>
> Based on the Rust standard library implementation, ver 1.62.0,
> licensed under "Apache-2.0 OR MIT", from:
>
>     https://github.com/rust-lang/rust/tree/1.62.0/library/alloc/src
>
> For copyright details, please see:
>
>     https://github.com/rust-lang/rust/blob/1.62.0/COPYRIGHT
>
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>

>  rust/kernel/sync/arc.rs | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a94e303217c6..752bd7c4699e 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -22,6 +22,7 @@ use crate::{
>  };
>  use alloc::boxed::Box;
>  use core::{
> +    any::Any,
>      fmt,
>      marker::{PhantomData, Unsize},
>      mem::{ManuallyDrop, MaybeUninit},
> @@ -220,6 +221,27 @@ impl<T: 'static> ForeignOwnable for Arc<T> {
>      }
>  }
>  
> +impl Arc<dyn Any + Send + Sync> {
> +    /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
> +    // Based on the Rust standard library implementation, ver 1.62.0, which is
> +    // Apache-2.0 OR MIT.
> +    pub fn downcast<T>(self) -> core::result::Result<Arc<T>, Self>
> +    where
> +        T: Any + Send + Sync,
> +    {
> +        if (*self).is::<T>() {
> +            // SAFETY: We have just checked that the type is correct, so we can cast the pointer.
> +            unsafe {
> +                let ptr = self.ptr.cast::<ArcInner<T>>();
> +                core::mem::forget(self);
> +                Ok(Arc::from_inner(ptr))
> +            }
> +        } else {
> +            Err(self)
> +        }
> +    }
> +}
> +
>  impl<T: ?Sized> Deref for Arc<T> {
>      type Target = T;