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Understanding Rust Error E0499: Multiple Mutable Borrows

What is Rust Error E0499?

Rust Error E0499 occurs when a variable is borrowed as mutable more than once. In Rust, a variable can either have many immutable references or one mutable reference. This is a safety feature to prevent data races and undefined behavior.

Erroneous Code Example:

#![allow(unused)]
fn main() {
  let mut i = 0;
  let mut x = &mut i;
  let mut a = &mut i;
  x;
  // error: cannot borrow `i` as mutable more than once at a time
}

In this example, the variable 'i' is borrowed as mutable twice, leading to Rust Error E0499. To fix this error, you may only create one mutable reference or create multiple immutable references.

Correct Code Example:

#![allow(unused)]
fn main() {
  let mut i = 0;
  let mut x = &mut i; // ok!

  // or:
  let mut i = 0;
  let a = &i; // ok!
  let b = &i; // still ok!
  let c = &i; // still ok!
  b;
  a;
}

Here, 'i' is either borrowed once as mutable or multiple times as immutable. This example will compile and run successfully without error E0499.

More on References and Borrowing in Rust

Rust enforces a strict borrowing policy to ensure memory safety and eliminate data races. By only allowing a single mutable reference or multiple immutable references, Rust guarantees exclusive access to the data when it is being mutated, avoiding conflicts.

For a more in-depth understanding of references and borrowing in Rust, you can read the References and Borrowing section of the Rust Book.

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