所有权

let s = "hello";

{                      // s 在这里无效, 它尚未声明
    let s = "hello";   // 从此处起，s 是有效的

    // 使用 s
} 

fn main() {
    let mut s = String::from("hello");

    change(&mut s);
}

fn change(some_string: &mut String) {
    some_string.push_str(", world");
}

let mut s = String::from("hello");

let r1 = &mut s;
let r2 = &mut s;

println!("{}, {}", r1, r2);

$ cargo run 
   Compiling multiple_mutable_references v0.1.0 (/home/klose/Documents/programming/html/klose911.github.io/src/rust/src/ownership/multiple_mutable_references)
error[E0499]: cannot borrow `s` as mutable more than once at a time
 --> src/main.rs:5:14
  |
4 |     let r1 = &mut s;
  |              ------ first mutable borrow occurs here
5 |     let r2 = &mut s;
  |              ^^^^^^ second mutable borrow occurs here
6 | 
7 |     println!("{}, {}", r1, r2);
  |                        -- first borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0499`.
error: Could not compile `multiple_mutable_references`.

To learn more, run the command again with --verbose.

let mut s = String::from("hello");

{
    let r1 = &mut s;

} // r1 在这里离开了作用域，所以我们完全可以创建一个新的引用

let r2 = &mut s;

let mut s = String::from("hello");

let r1 = &s; // 没问题
let r2 = &s; // 没问题
let r3 = &mut s; // 大问题

println!("{}, {}, and {}", r1, r2, r3);

$ cargo run 
   Compiling both_mut_immut_references v0.1.0 (/home/klose/Documents/programming/html/klose911.github.io/src/rust/src/ownership/both_mut_immut_references)
error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable
 --> src/main.rs:6:14
  |
4 |     let r1 = &s; // 没问题
  |              -- immutable borrow occurs here
5 |     let r2 = &s; // 没问题
6 |     let r3 = &mut s; // 大问题
  |              ^^^^^^ mutable borrow occurs here
7 | 
8 |     println!("{}, {}, and {}", r1, r2, r3);
  |                                -- immutable borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0502`.
error: Could not compile `both_mut_immut_references`.

To learn more, run the command again with --verbose.

let mut s = String::from("hello");

let r1 = &s; // 没问题
let r2 = &s; // 没问题
println!("{} and {}", r1, r2);
// 此位置之后 r1 和 r2 不再使用

let r3 = &mut s; // 没问题
println!("{}", r3);

fn main() {
    let reference_to_nothing = dangle();
}

fn dangle() -> &String {
    let s = String::from("hello");

    &s
}

$ cargo run 
   Compiling dangling_reference v0.1.0 (/home/klose/Documents/programming/html/klose911.github.io/src/rust/src/ownership/dangling_reference)
error[E0106]: missing lifetime specifier
 --> src/main.rs:5:16
  |
5 | fn dangle() -> &String {
  |                ^ help: consider giving it a 'static lifetime: `&'static`
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from

error: aborting due to previous error

For more information about this error, try `rustc --explain E0106`.
error: Could not compile `dangling_reference`.

To learn more, run the command again with --verbose.

fn dangle() -> &String { // dangle 返回一个字符串的引用

    let s = String::from("hello"); // s 是一个新字符串

    &s // 返回字符串 s 的引用
} // 这里 s 离开作用域并被丢弃。其内存被释放。
  // 危险！

fn dangle() -> String { 

    let s = String::from("hello"); 

    s 
} 

let s = String::from("hello world");

let hello = &s[0..5];
let world = &s[6..11];

let s = String::from("hello");

let slice = &s[0..2];
let slice = &s[..2];

let s = String::from("hello");

let len = s.len();

let slice = &s[3..len];
let slice = &s[3..];

let s = String::from("hello");

let len = s.len();

let slice = &s[0..len];
let slice = &s[..];

fn first_word(s: &String) -> &str {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }

    &s[..]
}

fn second_word(s: &String) -> &str {

fn main() {
    let mut s = String::from("hello world");

    let word = first_word(&s);

    s.clear(); // 错误!

    println!("the first word is: {}", word);
}

error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable
 --> src/main.rs:6:5
  |
4 |     let word = first_word(&s);
  |                           -- immutable borrow occurs here
5 | 
6 |     s.clear(); // 错误!
  |     ^^^^^^^^^ mutable borrow occurs here
7 | 
8 |     println!("the first word is: {}", word);
  |                                       ---- immutable borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0502`.
error: Could not compile `string_slice`.

To learn more, run the command again with --verbose.

let a = [1, 2, 3, 4, 5];

let a = [1, 2, 3, 4, 5];

let slice = &a[1..3];