error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable --> $DIR/writing-to-immutable-vec.rs:3:5 | LL | v[1] = 4; | ^ cannot borrow as mutable | help: consider changing this to be mutable | LL | let mut v: Vec = vec![1, 2, 3]; | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.