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

[Kent Gibson <warthog618@gmail.com>]

On Thu, Nov 24, 2022 at 04:15:01PM +0530, Viresh Kumar wrote:
> On 23-11-22, 19:37, Bartosz Golaszewski wrote:
> > 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)?
> 
> https://github.com/vireshk/libgpiod brgl/fix
> 
> For the benefit of others, I am pasting the entire diff of Rust changes required
> to make the C library enums named.
> 
> The part that can be improved, but I am not sure how, is the Error enum. Maybe
> Miguel or Kent can help ?
> 
> The problem is that the InvalidEnumValue Error needs to be generic, which makes
> it:
> 
> "
> pub enum Error<E> {
>     ...
>     InvalidEnumValue(&'static str, E),
> };
> 
> pub type Result<T, E> = std::result::Result<T, Error<E>>;
> "
> 
> Where E can be i32 or u32. Currently I just cast it everywhere as i32 to make
> it work.
> 

I don't see this as a problem for generics.  Whether the enum is signed
or unsigned doesn't need to affect the Error variant, much less the whole
Error type.  The Error doesn't need to respresent the type of the source
of the error, it needs to represent the type required to convey
information to the user.
Just accepting that the InvalidEnumValue variant expects i32, and casting
from u32 if necessary, seems appropriate to me.  Unless there are some
extreme values you are concerned about - but then you always switch it
up to  i64 ;-).

What is the problem that generics solve - that a subsequent bindgen or
gpiod.h change might change signage on you?  If so then cast them all
- even if the cast isn't necessary at present.

Cheers,
Kent.