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

[Zhao Liu]

On Tue, Dec 17, 2024 at 05:40:14PM +0100, Paolo Bonzini wrote:
> Date: Tue, 17 Dec 2024 17:40:14 +0100
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: Re: [PATCH 26/26] rust: callbacks: allow passing optional
>  callbacks as ()
> 
> Il mar 17 dic 2024, 16:55 Zhao Liu <zhao1.liu@intel.com> ha scritto:
> 
> > 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>.
> >
> 
> You wouldn't use Option<callback> at all, using () instead of None; the
> difference is that () does not have a parameter while None does (and the
> compiler cannot infer it). But I am okay with leaving this patch behind
> until there's a need.

Am I using the wrong terminology? Function pointers in a structure should
be called a vtable, rather than callbacks (for example, methods in TypeInfo,
read/write methods in MemoryRegionOps). Callbacks are typically function
pointers used as function parameters (for example, timer/gpio). So, is the
callback implementation here only used for the latter case?

Thanks,
Zhao