Error

"overflow evaluating the requirement" — How to Fix

Compiler Errors
2026-04-17T12:05:42+00:00
Fix the 'overflow evaluating the requirement' error by replacing one Rc pointer in the cycle with a Weak pointer to break the reference loop.

The error occurs because a and b form a reference cycle where each Rc<List> points to the other, preventing their reference counts from ever reaching zero. To fix this, replace one of the Rc<List> pointers in the cycle with a Weak<List> pointer so the cycle can be broken and memory cleaned up.

use std::cell::RefCell;
use std::rc::{Rc, Weak};

#[derive(Debug)]
enum List {
    Cons(i32, RefCell<Weak<List>>),
    Nil,
}

impl List {
    fn tail(&self) -> Option<&RefCell<Weak<List>>> {
        match self {
            Cons(_, item) => Some(item),
            Nil => None,
        }
    }
}

fn main() {
    let a = Rc::new(Cons(5, RefCell::new(Rc::downgrade(&Rc::new(Nil)))));
    let b = Rc::new(Cons(10, RefCell::new(Rc::clone(&a))));

    if let Some(link) = a.tail() {
        *link.borrow_mut() = Rc::downgrade(&b);
    }

    println!("a next item = {:?}", a.tail());
}