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

**Code:** `E0502` · **Category:** Rust · **Seen on:** Rust

You tried to mutate a value while an immutable reference to it is still in use.

## Meaning

Rust allows many readers or one writer at a time. Holding a reference into a Vec (e.g. from .first() or an index) and then pushing to it is the most common case.

## Common causes

- Holding a reference (iterator, slice, &v[0]) while mutating
- Calling a &mut self method while a borrow is alive
- Long-lived borrows in loops

## Quick fix

1. Finish using the immutable reference before mutating
2. Copy/clone the needed value out first
3. Collect changes and apply them after the loop

## Detailed fix by platform

### Rust

1. 

```rust
let first = v[0];          // copy the value (i32 is Copy)
v.push(4);
println!("{first}");
```


## How to diagnose

1. **Borrows** — Where is the immutable borrow used later?
2. **Loop** — Mutating what you iterate?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/rust/e0502-cannot-borrow-as-mutable)