rustScope errorMedium

Double mutable borrow panic with RefCell

Time: 0:00

BugFind the bug and fix the code

What it should do

Update RefCell value twice and return updated total. Example: refcell_double_borrow(10) -> 30

What happens instead

panicked at 'already borrowed: BorrowMutError'

use std::cell::RefCell;

fn refcell_double_borrow(initial: i32) -> i32 {
    let cell = RefCell::new(initial);
    let mut b1 = cell.borrow_mut();
    *b1 += 10;
    // Calling borrow_mut again while b1 is still alive panics
    let mut b2 = cell.borrow_mut();
    *b2 += 10;
    *b2
}

fn main() {
    println!("{}", refcell_double_borrow(10));
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the scope and closure bugs pattern

Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.

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use std::cell::RefCell; fn refcell_double_borrow(initial: i32) -> i32 { let cell = RefCell::new(initial); let mut b1 = cell.borrow_mut(); *b1 += 10; // Calling borrow_mut again while b1 is still alive panics let mut b2 = cell.borrow_mut(); *b2 += 10; *b2 } fn main() { println!("{}", refcell_double_borrow(10)); }

Double mutable borrow panic with RefCell

What it should do

What happens instead

More scope and closure bugs to practise

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Double mutable borrow panic with RefCell

What it should do

What happens instead

More scope and closure bugs to practise

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