Error code E0067

An invalid left-hand side expression was used on an assignment operation.

Erroneous code example:

#![allow(unused)]
fn main() {
12 += 1; // error!
}

You need to have a place expression to be able to assign it something. For example:

#![allow(unused)]
fn main() {
let mut x: i8 = 12;
x += 1; // ok!
}