Re: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for `Sync`

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

On 5/23/23 11:44, Alice Ryhl wrote:
> The safety comment on `impl Sync for Arc` references the Send safety
> comment. This commit avoids that in case the two comments drift apart in
> the future.
> 
> Suggested-by: Andreas Hindborg <a.hindborg@samsung.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> [...]
>   
> -// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync` for the
> -// same reason as above. `T` needs to be `Send` as well because a thread can clone an `&Arc<T>`
> -// into an `Arc<T>`, which may lead to `T` being accessed by the same reasoning as above.
> +// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `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 a `&Arc<T>` may clone it and get an
> +// `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
> +// example, when the reference count reaches zero and `T` is dropped.
>   unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
>   
>   impl<T> Arc<T> {

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>