borrowed data cannot be stored outside of its closure

Question

I have a World struct which contains a vector storing closures which capture references to the canvas attribute from World. I feel I have made all the lifetime notations that has to be given. but it throws an error which I am unable to understand.

I have shortened the example I have been working on:

use std::cell::RefCell;
use std::rc::Rc;
use wasm_bindgen::prelude::*;
use wasm_bindgen::JsCast;
struct Canvas {}
struct World<'a> {
    animations: Vec<AnimateClosure<'a>>,
    canvas: Canvas,
}
impl<'a> World<'a> {
    fn new(arg: &str) -> World {
        World {
            canvas: Canvas {},
            animations: vec![],
        }
    }
    fn check_collision(&'a mut self) {
        self.animation_cycle();
    }
    fn animation_cycle(&'a mut self) {
        let canvas = &self.canvas;
        let width = 10.0;
        let height = 10.0;
        let closure = move |x: f64| {
            let new = canvas;
        };
        let animation = AnimateClosure::new(10, Box::new(closure) as Box<dyn Fn(f64)>);
        self.animations.push(animation);
    }
}
pub struct AnimateClosure<'a> {
    closure: Box<dyn Fn(f64) + 'a>,
}
impl<'a> AnimateClosure<'a> {
    pub fn new(duartion: u32, closure: Box<dyn Fn(f64) + 'a>) -> AnimateClosure<'a> {
        AnimateClosure { closure }
    }
}

pub fn main() {
    let mut world = World::new("game-of-drops");
    let closure_cell = Rc::new(RefCell::new(None));
    let closure_cell_clone = Rc::clone(&closure_cell);
    *closure_cell.borrow_mut() = Some(Closure::wrap(Box::new(move |x: f64| {
        world.check_collision();
        world.animation_cycle();
    }) as Box<dyn FnMut(_)>));
}

I feel the problem might be from the canvas field stored in animate_closure, but I do not understand how to solve the problem with more lifetime annotations.

error: borrowed data cannot be stored outside of its closure
  --> src/load.rs:45:9
   |
41 |     let mut world = World::new("game-of-drops");
   |         --------- ...so that variable is valid at time of its declaration
...
44 |     *closure_cell.borrow_mut() = Some(Closure::wrap(Box::new(move |x: f64| {
   |                                                              ------------- borrowed data cannot outlive this closure
45 |         world.check_collision();
   |         ^^^^^ --------------- cannot infer an appropriate lifetime...
   |         |
   |         cannot be stored outside of its closure

Cargo.toml

wasm-bindgen = "0.2.48"

[dependencies.web-sys]
version = "0.3.25"
features = ['console', 'CanvasRenderingContext2d']

Edit

This is not a duplicate of the question. I am not passing any reference to the closure to cause any conflicts here. Only reference comes from the attribute of world canvas.

It looks like your question might be answered by the answers of [What does “borrowed data cannot be stored outside of its closure” mean?](https://stackoverflow.com/q/51118023/155423). If not, please **[edit]** your question to explain the differences. Otherwise, we can mark this question as already answered. — Shepmaster, Aug 27 '19 at 15:23
Possible duplicate of [What does "borrowed data cannot be stored outside of its closure" mean?](https://stackoverflow.com/questions/51118023/what-does-borrowed-data-cannot-be-stored-outside-of-its-closure-mean) — E_net4, Aug 27 '19 at 15:30
Duplicate may need to be combined with [What is the difference between '&self' and '&'a self'?](https://stackoverflow.com/q/45833618/155423). — Shepmaster, Aug 27 '19 at 16:34
@Shepmaster by having ``(&'a mut self)`` i was trying to say that the ``self`` would only live for ``World<'a>`` lifetime. As the example you shared itseems that can constrain the lifetime if the struct is Contravariant. Should we always prefer a new lifetime ``b`` for all methods? What would be an example where only the same lifetime ``a`` make sense in method defenition? — raj, Aug 27 '19 at 20:41

borrowed data cannot be stored outside of its closure

0 Answers0