可变与不可变

• 不可变
• 可变
• 不可变借用
• 可变借用

let x = 5;
x = 6;  // error

let mut x = 5;
x = 6

let mut x = 5;
let y = &mut x;

let mut x = 5;
let mut y = &mut x;

let (mut x,y) = (5,6)
fn foo(mut x:i32) {}