Re: libgpiod: rust bindings and bindgen issue with C enums

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Tue, Nov 22, 2022 at 8:08 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
> On Tue, Nov 22, 2022 at 5:55 PM Miguel Ojeda
> <miguel.ojeda.sandonis@xxxxxxxxx> wrote:
> >
> > On Tue, Nov 22, 2022 at 4:38 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
> > >
> > > ---
> > > The expression that defines the value of an enumeration constant shall
> > > be an integer constant expression that has a value representable as an
> > > int.
> > > ---
> > >
> > > and virtually all compilers store enum variables as signed integers,
> >
> > I don't think this is true. Both GCC and Clang seem to pick an
> > unsigned one if possible (for the enum, not the constants), e.g.
> > https://godbolt.org/z/6zjzMdP3T. I assume bindgen is using the one
> > decided by clang.
> >
> > Note that the quote of the standard is a constraint, i.e. the values
> > of the constants need to fit in an `int` (and the compiler is required
> > to issue a diagnostic if they don't, under `-Wpedantic` in GCC/Clang).
> >
> > > As enums are naturally signed integers in the C world - can we somehow
> > > make bindgen default to c_int for all enum types?
> >
> > This would be https://github.com/rust-lang/rust-bindgen/issues/1966,
> > where it has been suggested as an option (as well as the fact that the
> > constants are not being generated as `c_int`).
> >
>
> IIUC this is not done yet.
>
> Viresh' code makes the assumption that all Enums are unsigned
> integers. This would be easy to just convert to signed ints but having
> both types supported seems impossible. For instance there's the
> InvalidEnumValue error that takes an u32. There are more instances
> where the enum's type matters.
>
> What should I do in this case to accommodate two types for enums? I'm
> bad at rust so I don't even know. In C++ I'd probably use templates if
> I had this problem. So generics?
>
> Bart

Viresh,

Could you take a look at https://github.com/brgl/libgpiod-private?
There's a branch called topic/further-libgpiod-v2-updates. Can you
check out commit 5a4e08d546a8ec32757e6c9cc59d7a16939721ea and tell me
how you'd make rust bindings work with it because I'm out of ideas
(and my comfort zone)?

Thanks in advance,
Bartosz