一、变量和可变性

变量默认是不可改变的（immutable）。这是 Rust 提供给你的众多优势之一，让你得以充分利用 Rust 提供的安全性和简单并发性来编写代码。不过，你仍然可以使用可变变量。
文件名：src/main.rs

fn main() {
   let x=5;
   println!(" x 的值是: {}",x);
   x=6;
   println!(" x 的值是: {}",x);
}

保存并使用cargo run运行，会看到报错信息

 ~/myrust/guessing_game   master  cargo run
   Compiling guessing_game v0.1.0 (/Users/shuai/myrust/guessing_game)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:6:4
  |
4 |    let x=5;
  |        -
  |        |
  |        first assignment to `x`
  |        help: consider making this binding mutable: `mut x`
5 |    println!(" x 的值是: {}",x);
6 |    x=6;
  |    ^^^ cannot assign twice to immutable variable

For more information about this error, try `rustc --explain E0384`.
error: could not compile `guessing_game` due to previous error

尽管变量默认是不可变的，你仍然可以在变量名前添加 mut 来使其可变

fn main() {
   let mut x=5;
   println!(" x 的值是: {}",x);
   x=6;
   println!(" x 的值是: {}",x);
}

二、隐藏

我们可以定义一个与之前变量同名的新变量。Rustacean 们称之为第一个变量被第二个 隐藏（Shadowing） 了，这意味着当您使用变量的名称时，编译器将看到第二个变量。

fn main() {
    let x = 5;

    let x = x + 1;

    {
        let x = x * 2;
        println!("The value of x in the inner scope is: {x}");
    }

    println!("The value of x is: {x}");
}

运行结果

 ✘  ~/myrust/guessing_game   master  cargo run
   Compiling guessing_game v0.1.0 (/Users/shuai/myrust/guessing_game)
    Finished dev [unoptimized + debuginfo] target(s) in 0.34s
     Running `target/debug/guessing_game`
The value of x in the inner scope is: 12
The value of x is: 6

mut 与隐藏的另一个区别是，当再次使用 let 时，实际上创建了一个新变量，我们可以改变值的类型，并且复用这个名字。

fn main() {
    let spaces = "   ";
    let spaces = spaces.len();
}

然而，如果尝试使用 mut，将会得到一个编译时错误，如下所示：

fn main() {
    let mut spaces = "   ";
    spaces = spaces.len();
}

上述代码会报如下错误：

 ~/myrust/guessing_game   master  cargo run
   Compiling guessing_game v0.1.0 (/Users/shuai/myrust/guessing_game)
error[E0308]: mismatched types
 --> src/main.rs:5:14
  |
4 |     let mut spaces = "   ";
  |                      ----- expected due to this value
5 |     spaces = spaces.len();
  |              ^^^^^^^^^^^^ expected `&str`, found `usize`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `guessing_game` due to previous error