Re: [PATCH 02/14] rust: add basic QObject bindings

[Zhao Liu <zhao1.liu@intel.com>]

> > > +pub struct QObject(&'static UnsafeCell<bindings::QObject>);
> > 
> > It seems Opaque<> feels more natural than UnsafeCell<>.
> > 
> > Opaque::from_raw() requires *mut T, but QObject::from_raw() and
> > QObject::clone_from_raw() mainly play with C bindings which usually use
> > *mut pointer. So it seems unnecessary to convert *mut to *const in the
> > middle.
> > 
> > And furthermore, I think QObject(Opaque<bindings::QObject>) is better
> > than QObject(&'static Opaque<bindings::QObject>). From a semantic view,
> > C's QObject is a struct, while Rust's QObject is a reference, which seems
> > somewhat mismatched.
> > 
> > I'm not sure yet if there may be gaps when remove &'static, but it
> > looks like using &'static Opaque<> instead of &'static UnsafeCell<> is
> > Okay in code?
> 
> I am using UnsafeCell because the QObject here is always valid, i.e.
> MaybeUninit is explicitly not necessary.  Opaque explicitly allows it to be
> invalid, here instead the API is "create via C code or FFI and only then
> create the QObject".

Just want to clarify the use for Opaque, we've already used Opaque<>::from_raw()
for the case "create via C code or FFI and only then create the" something? In
DeviceState::init_clock_[in|out](), and ObjectClassMethods::new().

But I also agree, for this case MaybeUninit is not necessary. And PhantomPinned
is simply replaced by &'static.

> However, while it is possible to use Opaque<> instead of UnsafeCell<>, it is
> not possible to make this a simple wrapper because QObject is unmovable and
> reference counted.  That is, QObject is the equivalent of (for example)
> Owned<DeviceState>.

Got it, if Rust owns data by QObject(Opaque<>) but without Owned<>, this
can't work since it's no way to handle refcnt.

So &'static UnsafeCell<> is the simplest way to meet both unmovable &
refcnt rquirements.

I'm good with this design. Thanks!

Regards,
Zhao