变量的引用与克隆

# 变量的引用与克隆
## 一个变量引用后不可以被修改
源码如下，s1 是一个mutable变量，但是在s2引用s1以后，如果修改s1，会panic
```rust
fn main() { 
    let mut s1 = String::from("abc");
    let mut s2 = &s1;
    println!("{s2}");
    s1.push_str("bcd");
    println!("s2: {s2},s1:  {s1}");
}
```

执行输出：
```shell
➜  ownership git:(master) ✗ cargo run 
   Compiling ownership v0.1.0 (/Users/icm/RustC/ownership)
warning: variable does not need to be mutable
  --> src/main.rs:54:6
   |
54 |     let mut s2 = &s1;
   |         ----^^
   |         |
   |         help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

error[E0502]: cannot borrow `s1` as mutable because it is also borrowed as immutable
  --> src/main.rs:56:2
   |
54 |     let mut s2 = &s1;
   |                  --- immutable borrow occurs here
55 |     println!("{s2}");
56 |     s1.push_str("bcd");
   |     ^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
57 |     println!("s2: {s2},s1:  {s1}");
   |                   ---- immutable borrow later used here

For more information about this error, try `rustc --explain E0502`.
warning: `ownership` (bin "ownership") generated 1 warning
error: could not compile `ownership` (bin "ownership") due to previous error; 1 warning emitted
```