rust/src/librustc_error_codes/error_codes/E0499.md

A variable was borrowed as mutable more than once.

Erroneous code example:

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

Please note that in Rust, you can either have many immutable references, or one mutable reference. For more details you may want to read the References & Borrowing section of the Book.

Example:

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;