Cannot Borrow Data In A Reference As Mutable

Rust: Cannot borrow immutable reference as mutable

Problem Description

You are trying to mutate a reference that is immutable. In Rust, a mutable reference is not directly a mutable reference to an object but a unique reference to a mutable location.

Example

 fn change_str(s: &mut str) {     // ... }  fn main() {     let s = "immutable";     change_str(&mut s); // error: cannot borrow immutable borrowed content as mutable } 

In this example, the type of s is actually str, which means the reference you are passing to change_str is mut str. This is not allowed because s is immutable.

Solution

To fix the code, you can either make s mutable or use a mutable reference to the underlying data. For example:

 fn change_str(s: &mut str) {     // ... }  fn main() {     let mut s = "mutable";     change_str(&mut s); // okay } 

Alternatively, you can use a mutable reference to the underlying data:

 fn change_str(s: &mut str) {     // ... }  fn main() {     let s = "immutable";     change_str(&mut s.as_mut_str()); // okay }