How do I pass an array to a function in Rust and change its content?

I want to pass an array to a function and change the content inside it:
fn change_value(mut arr: &[i32]) {
arr[1] = 10;
}

fn main() {
let mut arr: [i32; 4] = [1, 2, 3, 4];
change_value(&arr);
println!("this is {}", arr[1]);
}


I'm getting this error:
warning: variable does not need to be mutable
--> src/main.rs:2:17
|
2 | fn change_value(mut arr: &[i32]) {
| ----^^^
| |
| help: remove this mut
|
= note: #[warn(unused_mut)] on by default

error[E0594]: cannot assign to arr[_] which is behind a & reference
--> src/main.rs:3:5
|
2 | fn change_value(mut arr: &[i32]) {
| ------ help: consider changing this to be a mutable reference: &mut [i32]
3 | arr[1] = 10;
| ^^^^^^^^^^^ arr is a & reference, so the data it refers to cannot be written

warning: variable does not need to be mutable
--> src/main.rs:7:9
|
7 | let mut arr: [i32; 4] = [1, 2, 3, 4];
| ----^^^
| |
| help: remove this mut


I've been searching around, but I can't find anything.

Rust references (denoted by the & sign) are of two kinds: immutable (&T) and mutable (&mut T). In order to change the value behind the reference, this reference has to be mutable, so you need to:

• accept &mut [i32] as the function argument, not &[i32]

• pass &mut arr to the function, not &arr:

fn change_value(arr: &mut [i32]) {
arr[1] = 10;
}

fn main() {
let mut arr: [i32; 4] = [1, 2, 3, 4];
change_value(&mut arr);
println!("this is {}", arr[1]);
}


You don't need mut arr in change_value's argument because mut there denotes mutability of that variable, not of the data it points to. With mut arr: &[i32] you can reassign arr itself (for it to point to a different slice), but you can't change the data it references.

If you wanted to accept an array instead of a slice, you could also do that:
fn change_value(arr: &mut [i32; 4]) {
arr[1] = 10;
}


See also:

• What's the difference between placing "mut" before a variable name and after the ":"?

Stack Overflow Q#24831828, score: 89