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To:     Viresh Kumar <viresh.kumar@linaro.org>
From:   =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
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Subject: Re: [PATCH V7 0/8] libgpiod: Add Rust bindings
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On Tuesday, October 25th, 2022 at 08:42, Viresh Kumar <viresh.kumar@linaro.=
org> wrote:


> On 21-10-22, 14:34, Bj=C3=B6rn Roy Baron wrote:
>=20
> > > impl<'b> Event<'b> {
> > >=20
> > > /// Get an event stored in the buffer.
> > > - pub(crate) fn new(buffer: &'b Buffer, index: usize) -> Result<Self>=
 {
> > >=20
> > > + pub(crate) fn new(buffer: &'b Buffer, index: usize) -> Result<Event=
<'b>> {
> >=20
> > This looks good to me.
>=20
> > > +impl<'e, 'b> Event<'e> {
> > >=20
> > > + pub fn event_clone(event: &Event<'b>) -> Result<Event<'e>>
> > >=20
> > > + where
> > > + 'e: 'b,
> >=20
> > Using `Event<'b>` on both sides should work fine. `Event` is
> > covariant in it's lifetime parameter, so `Event<'b>` can be turned
> > into `Event<'e>` with `'e` being a shorter lifetime than `'b`. What
> > you wrote here is not incorrect, so if you prefer keeping it this
> > way that is fine with me.
>=20
>=20
> That doesn't let the cloned event to live past read_edge_events().
>=20
> error[E0502]: cannot borrow `buffer` as mutable because it is also borrow=
ed as immutable
> --> libgpiod/examples/gpio_events.rs:70:50
>=20
> |
> 64 | let event =3D buffer.event(0)?;
> | --------------- immutable borrow occurs here
> ...
> 70 | let count =3D request.read_edge_events(&mut buffer)?;
> | ^^^^^^^^^^^ mutable borrow occurs here
> ...
> 86 | }
> | - immutable borrow might be used here, when `cloned_event` is dropped a=
nd runs the `Drop` code for type `libgpiod::request::Event`
>=20
> --
> viresh

I would have expected that to work fine, but as it doesn't work keeping you=
r code with two separate lifetimes is fine.

Cheers,
Bjorn