error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable

Rust’s borrow checker prevents you from having mutable and immutable references at the same time.

Fix 1: Limit the scope of borrows

let mut data = vec![1, 2, 3];

// ❌ Immutable borrow lives too long
let first = &data[0];
data.push(4);  // Error! Can't mutate while borrowed
println!("{}", first);

// ✅ Use the immutable borrow before mutating
let first = &data[0];
println!("{}", first);  // Done with immutable borrow
data.push(4);  // Now OK

Fix 2: Clone the data

let mut data = vec![1, 2, 3];
let first = data[0].clone();  // Own the value
data.push(4);  // Fine — no borrow conflict
println!("{}", first);

Fix 3: Use interior mutability

use std::cell::RefCell;

let data = RefCell::new(vec![1, 2, 3]);
data.borrow_mut().push(4);

Related resources

• Rust cheat sheet

Related: Rust: Expected Type X, Found Y — How to Fix It

Related: Rust: Value Used After Move — How to Fix It