$ rustup update stable

$ vim main.rs

fn main() {
    println!("Hello, world!");
}

$ rustc main.rs
$ ./main
Hello, world!

$ cargo new hello_cargo --bin
$ cd hello_cargo
$ vim src/main.rs

fn main() {
    println!("Hello, world!");
}

$ cargo build # 建置
$ cargo run   # 建置並執行

fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}

error[E0384]: re-assignment of immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         - first assignment to `x`
3 |     println!("The value of x is: {}", x);
4 |     x = 6;
  |     ^^^^^ re-assignment of immutable variable