一 创建项目

cargo new variables

二 变量可变性的实例错误代码

vim ./src/main.rs

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

三 编译查看错误提示

cargo run

cargo run 
   Compiling variables v0.1.0 (/home/li/projects/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         -
  |         |
  |         first assignment to `x`
  |         help: make this binding mutable: `mut x`
3 |     println!("The value of x is: {}", x);
4 |     x = 6;
  |     ^^^^^ cannot assign twice to immutable variable

error: aborting due to previous error

小结：默认情况，let定义的变量是不可变的，如果可变需加关键字mut

四 增加mut关键字，让变量可变

vim ./src/main.rs

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

输出

cargo run 
   Compiling variables v0.1.0 (/home/li/projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 4.98s
     Running `target/debug/variables`
The value of x is: 5
The value of x is: 6

五 常量

使用const关键字定义常量
const MAX_POINTS: u32 = 100_000; //在数字字面值下面插入下划线来提高数字可读性，这里等价于 100000

六 隐藏（shadowing）变量

重复使用let关键字，定义同名变量来隐藏之前的变量。
实际上是创建一个新的变量。
可改变变量的类型和值

fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    let x = "6+1";
    println!("The value of x is: {}", x);
}

如果使用mut，则会报编译错误
因为mut不允许改变变量类型

fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = "6+1";
    println!("The value of x is: {}", x);
    
}

cargo run
   Compiling variables v0.1.0 (/home/li/projects/variables)
error[E0308]: mismatched types
 --> src/main.rs:4:9
  |
4 |     x = "6+1";
  |         ^^^^^ expected integer, found reference
  |
  = note: expected type `{integer}`
             found type `&'static str`

七总结

1 使用大型数据结构时，适当地使用可变变量
2 对于较小的数据结构，总是创建新实例
3 了解let，mut，const关键字用法