error[E0502]: cannot borrow as mutable because it is also borrowed as immutable — how to fix

error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
 --> src/main.rs:4:5
  |
3 |     let first = &v[0];
  |                  - immutable borrow occurs here
4 |     v.push(4);
  |     ^^^^^^^^^ mutable borrow occurs here
5 |     println!("{first}");
  |               ------- immutable borrow later used here

Quick fix: End the immutable borrow before mutating: use first before v.push(4), or copy the value out (let first = v[0]; for Copy types, .clone() otherwise).

What it means

Rust allows either many shared (&) references or one mutable (&mut) reference at a time. Here a shared reference (first) is still used later, while you try to mutate v. That rule exists for a real reason: push may reallocate the vector, which would leave first pointing at freed memory.

Fix 1: use the reference before mutating

let first = &v[0];
println!("{first}");
v.push(4);              // OK: `first` is no longer used

Fix 2: copy the value out

let first = v[0];       // i32 is Copy
// or: let first = v[0].clone();
v.push(4);
println!("{first}");

Fix 3: modifying a collection while iterating

for x in &v {
    if *x > 2 { v.push(*x); }   // E0502
}

Collect first, then mutate:

let extra: Vec<_> = v.iter().filter(|x| **x > 2).copied().collect();
v.extend(extra);

To remove items, use v.retain(|x| ...). To modify in place, use for x in v.iter_mut().

Fix 4: struct methods

Calling self.helper() (which borrows all of self) while holding a reference to self.items triggers this too. Borrow only the fields you need, or pass them as arguments to a free function.

When the borrow really must be shared

For shared mutable state, use interior mutability: RefCell (single thread) or Mutex/RwLock (threads).

Last updated 2026-10-27.