Hi !

I'm entering in the lifetime world. Until today I had avoided them. But, I have to face them in generic rewrite of my code.

There is a reproduction of my problem:

use std::sync::{Arc, RwLock};

struct SoldierId(usize);
struct Soldier;
struct World(Vec<Soldier>);
struct State(Arc<RwLock<World>>);
struct Context {
    state: Arc<State>,
}

trait IntoSubject<'a, T> {
    fn into_subject(&self, world: &'a World) -> &'a T;
}

impl<'a> IntoSubject<'a, Soldier> for SoldierId {
    fn into_subject(&self, world: &'a World) -> &'a Soldier {
        &world.0[self.0]
    }
}

struct Processor<'a> {
    ctx: &'a Context,
}

impl<'a> Processor<'a> {
    fn process<I, T: 'a>(&self, i: I)
    where
        I: IntoSubject<'a, T>,
    {
        let world = self.ctx.state.0.read().unwrap();
        let _subject = i.into_subject(&world);
    }
}

fn main() {
    let objects = vec![];
    let state = Arc::new(State(Arc::new(RwLock::new(World(objects)))));
    let ctx = Context { state };
    let processor = Processor { ctx: &ctx };
    processor.process(SoldierId(0));
}

error[E0597]: `world` does not live long enough
  --> src/main.rs:31:39
   |
25 | impl<'a> Processor<'a> {
   |      -- lifetime `'a` defined here
...
30 |         let world = self.ctx.state.0.read().unwrap();
   |             ----- binding `world` declared here
31 |         let _subject = i.into_subject(&world);
   |                        ---------------^^^^^^-
   |                        |              |
   |                        |              borrowed value does not live long enough
   |                        argument requires that `world` is borrowed for `'a`
32 |     }
   |     - `world` dropped here while still borrowed

In this context, my subject (T) is owned by World. So, in Process::process which contain World (thought State then Context), the life of T must be the same as World. So, I specfy 'a. Error say &world is dropped (l. 32) but still borrowed. How to indicate than T no need to be dropped after world ? Maybe I'm talking nonsense

Thanks in advance !

2 posts - 2 participants

Read full topic