Re: [PATCH 26/26] rust: callbacks: allow passing optional callbacks as ()

[Zhao Liu]

On Mon, Dec 09, 2024 at 01:37:17PM +0100, Paolo Bonzini wrote:
> Date: Mon,  9 Dec 2024 13:37:17 +0100
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH 26/26] rust: callbacks: allow passing optional callbacks as
>  ()
> X-Mailer: git-send-email 2.47.1
> 
> In some cases, callbacks are optional.  Using "Some(function)" and "None"
> does not work well, because when someone writes "None" the compiler does
> not know what to use for "F" in "Option<F>".

I understand the direct use case is MemoryRegionOps, which has optional
callbacks. However, I'm not quite sure how exactly it should be applied
to C bindings and how it will play with Option<callback>.

Could u pls provide a simple example?

> Therefore, adopt () to mean a "null" callback.  It is possible to enforce
> that a callback is valid by adding a "let _: () = F::ASSERT_IS_SOME" before
> the invocation of F::call.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  rust/qemu-api/src/callbacks.rs | 97 ++++++++++++++++++++++++++++++++++
>  1 file changed, 97 insertions(+)
> 
> diff --git a/rust/qemu-api/src/callbacks.rs b/rust/qemu-api/src/callbacks.rs
> index 6401d807198..83c681d6478 100644
> --- a/rust/qemu-api/src/callbacks.rs
> +++ b/rust/qemu-api/src/callbacks.rs
> @@ -76,6 +76,31 @@
>  /// call_it(&move |_| String::from(x), "hello workd");

typo: s/workd/word/ (in previous patch :-))