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! }