Re: [libgpiod][PATCH 15/16] bindings: rust: make mutators return &mut self

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Mon, Jan 16, 2023 at 10:41 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> On 16-01-23, 09:42, Bartosz Golaszewski wrote:
> > On Mon, Jan 16, 2023 at 7:02 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> > >
> > > On 13-01-23, 22:52, Bartosz Golaszewski wrote:
> > > > diff --git a/bindings/rust/libgpiod/tests/line_request.rs b/bindings/rust/libgpiod/tests/line_request.rs
> > > > index c3fc37b..561f4e8 100644
> > > > --- a/bindings/rust/libgpiod/tests/line_request.rs
> > > > +++ b/bindings/rust/libgpiod/tests/line_request.rs
> > > > @@ -97,13 +97,11 @@ mod line_request {
> > > >              config.lconfig_add_settings(&offsets);
> > > >              config.request_lines().unwrap();
> > > >
> > > > -            let request = config.request();
> > > > -
> > >
> > > Why remove this ? And open-code it ?
> > >
> >
> > Because I'm a Rust noob and couldn't figure out how to deal with the
> > mutable/non-mutable borrow vomit that ensued when I kept the local
> > variable. If you could improve upon this one, it would be great!
>
> From what I can understand, config.request() returns a mutable
> reference and you don't need a mutable variable to keep it, since the
> variable doesn't need to store another reference later. Just moving
> back to original code should work here.
>

Seems like I went overboard with converting all of those cases but in
same places this still fails:

error[E0502]: cannot borrow `config` as immutable because it is also
borrowed as mutable
   --> libgpiod/tests/line_request.rs:449:24
    |
441 |             let request = config.request();
    |                           ---------------- mutable borrow occurs here
...
449 |             let info = config.chip().line_info(0).unwrap();
    |                        ^^^^^^^^^^^^^ immutable borrow occurs here
...
456 |             request.reconfigure_lines(&lconfig).unwrap();
    |             ----------------------------------- mutable borrow
later used here

If I make config.chip() return &mut Chip then it fails like that:

error[E0499]: cannot borrow `config` as mutable more than once at a time
   --> libgpiod/tests/line_request.rs:449:24
    |
441 |             let request = config.request();
    |                           ---------------- first mutable borrow
occurs here
...
449 |             let info = config.chip().line_info(0).unwrap();
    |                        ^^^^^^^^^^^^^ second mutable borrow occurs here
...
456 |             request.reconfigure_lines(&lconfig).unwrap();
    |             ----------------------------------- first borrow
later used here

Not sure how to go about this.

Bart