Re: [PATCH V7 0/8] libgpiod: Add Rust bindings

[Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>]

On Tuesday, October 25th, 2022 at 08:42, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:


> On 21-10-22, 14:34, Björn Roy Baron wrote:
> 
> > > impl<'b> Event<'b> {
> > > 
> > > /// Get an event stored in the buffer.
> > > - pub(crate) fn new(buffer: &'b Buffer, index: usize) -> Result<Self> {
> > > 
> > > + pub(crate) fn new(buffer: &'b Buffer, index: usize) -> Result<Event<'b>> {
> > 
> > This looks good to me.
> 
> > > +impl<'e, 'b> Event<'e> {
> > > 
> > > + pub fn event_clone(event: &Event<'b>) -> Result<Event<'e>>
> > > 
> > > + where
> > > + 'e: 'b,
> > 
> > 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.
> 
> 
> That doesn't let the cloned event to live past read_edge_events().
> 
> error[E0502]: cannot borrow `buffer` as mutable because it is also borrowed as immutable
> --> libgpiod/examples/gpio_events.rs:70:50
> 
> |
> 64 | let event = buffer.event(0)?;
> | --------------- immutable borrow occurs here
> ...
> 70 | let count = request.read_edge_events(&mut buffer)?;
> | ^^^^^^^^^^^ mutable borrow occurs here
> ...
> 86 | }
> | - immutable borrow might be used here, when `cloned_event` is dropped and runs the `Drop` code for type `libgpiod::request::Event`
> 
> --
> viresh

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

Cheers,
Bjorn