Trying to iterate through a mesh structure results in "borrowed value does not live long enough" - iterator

Question

I am trying to create a data structure that can store monitor location data.
The approach I'm taking is that each screen has references to 4 neighbors stored in a HashMap<Direction, Rc<RefCell<Screen>>>.
What I'm struggling with is how to "move"/iterate in a direction until I reach the edge of the mesh:
use std::cell::RefCell;
use std::collections::HashMap;
use std::rc::Rc;
#[derive(Debug, Copy, Clone, Default)]
pub struct ScreenCoord {
left: i32,
right: i32,
bottom: i32,
top: i32,
}
impl ScreenCoord {
fn new_primary_from_dimensions(width: i32, height: i32) -> ScreenCoord {
ScreenCoord {
left: 0,
right: width,
top: 0,
bottom: height,
}
}
}
pub struct ScreenNetwork {
primary_screen: Rc<RefCell<Screen>>,
}
impl ScreenNetwork {
pub fn new(width: i32, height: i32) -> ScreenNetwork {
ScreenNetwork {
primary_screen: Rc::new(RefCell::new(Screen::new(
ScreenCoord::new_primary_from_dimensions(width, height),
))),
}
}
pub fn add_screen(&mut self, new_width: i32, new_height: i32, direction: Direction) {
let mut new_scrn = Screen::new(ScreenCoord::new_primary_from_dimensions(
new_width, new_height,
));
let mut original_screen = &self.primary_screen;
while let Some(next) = original_screen.borrow().neighbours.get(&direction) {
original_screen = next;
}
// Do stuff with original_screen
// new_scrn.neighbours.insert(Direction::from_u8((direction.clone() as u8) ^ 0b11).unwrap(), original_screen.clone());
// original_screen.borrow_mut().neighbours.insert(direction, Rc::new(RefCell::new(new_scrn)));
}
}
/// Screen with information about adjacent screens
#[derive(Default, Debug)]
pub struct Screen {
neighbours: HashMap<Direction, Rc<RefCell<Screen>>>,
coordinates: ScreenCoord,
}
impl Screen {
pub fn new(coord: ScreenCoord) -> Screen {
Screen {
coordinates: coord,
neighbours: HashMap::new(),
}
}
}
#[derive(Debug, PartialEq, Hash, Eq, Clone)]
pub enum Direction {
Left = 0,
Top,
Bottom,
Right,
}
fn main() {}
playground
error[E0597]: borrowed value does not live long enough
--> src/main.rs:43:32
|
43 | while let Some(next) = original_screen.borrow().neighbours.get(&direction) {
| ^^^^^^^^^^^^^^^^^^^^^^^^ - temporary value dropped here while still borrowed
| |
| temporary value does not live long enough
...
51 | }
| - temporary value needs to live until here
I understand why the error is happening, but I have no clue as to how I can fix it. I tried to clone original_screen and next, but then the compiler complains that I am trying to set original_screen while still borrowing it.

Answer 1

You are using Rc, so take advantage of it. Generally speaking, taking a reference to a Rc is asking for problems.
First try: clone Rc when needed:
let mut original_screen = self.primary_screen.clone();
while let Some(next) = original_screen.borrow().neighbours.get(&direction) {
original_screen = next.clone();
}
It fails with the message:
cannot assign to original_screen because it is borrowed
True enough: having a borrow in the while condition will not let you update it, but it can be rewritten easily:
loop {
if let Some(next) = original_screen.borrow().neighbours.get(&direction) {
original_screen = next.clone()
} else {
break;
}
}
That code is equivalent, and indeed it fails with the same error message! But now we are closer to a solution:
loop {
let next = if let Some(next) = original_screen.borrow().neighbours.get(&direction) {
next.clone()
} else {
break;
};
original_screen = next;
}
Now it compiles because the assignment is done after the borrow is released.