Rust Error E0596: Mutably Borrowing a Non-Mutable Variable

Understanding Rust Error E0596

Rust Error E0596 occurs when you attempt to mutably borrow a non-mutable variable. This error is a result of the Rust language's strict rules about variable mutability and borrowing, which help to prevent many common programming errors such as data races and unintended side effects.

Example of Erroneous Code:

```rust
#![allow(unused)]
fn main() {
  let x = 1;
  let y = &mut x; // error: cannot borrow mutably
}
```

In the above example, the variable x is declared as non-mutable with the keyword 'let'. However, the code attempts to create a mutable borrow on x with '&mut x', resulting in Error E0596.

Fixing Rust Error E0596

To resolve Rust Error E0596, you need to explicitly declare the variable as mutable using the 'mut' keyword. This allows the variable to be modified and safely borrowed as mutable.

Corrected Code Example:

```rust
#![allow(unused)]
fn main() {
  let mut x = 1;
  let y = &mut x; // ok!
}
```

In this corrected example, the variable x is declared with the 'mut' keyword, allowing it to be borrowed mutably. The error is resolved, and the code will now compile successfully.

Further Discussion

Understanding Rust's strict borrowing rules and mutability features is crucial for implementing safe, efficient, and error-free code. These rules help to minimize the chance of data races, aliasing, and other potential problems that can arise from mutable variables and concurrency.

By practicing correct use of mutable and immutable variables, as well as borrowing and ownership, you will be better equipped to write clean, performant, and reliable Rust programs.