E0499 — Cannot Borrow as Mutable More Than Once

What causes this error

Rust's borrow checker prevents two mutable references to the same data from existing simultaneously. This prevents data races and ensures memory safety.

How to fix it

Restructure the code so only one mutable borrow exists at a time. Use a block to limit the scope of the first mutable borrow. Consider using Cell, RefCell, or Mutex for interior mutability patterns.

let mut s = String::new();
let r1 = &mut s;
let r2 = &mut s;