# Rust notes

## 错误处理

### shortcuts for panic on error: `unwrap` and `expect`

- `unwrap`
    - 对`Result<T, E>`
        - 如果`Ok`, 返回值
        - 如果`Err`, `panic!`

- `expect`
    - 错误时,提供一个消息并`panic!`

### propagating errors

使用`?`快速的替代多余代码返回错误.

一个示例:

```rust
fn read_username_from_file() -> Result<String, io::Error> {
    let username_file_result = File::open("hello.txt");
    let mut username_file = match username_file_result {
        Ok(file) => file,
        Err(e) => return Err(e),
    };
    let mut username = String::new();
    match username_file.read_to_string(&mut username) {
        Ok(_) => Ok(username),
        Err(e) => Err(e),
    }
}
```

使用`?`简单替换.

```rust
fn read_username_from_file() -> Result<String, io::Error> {
    let mut username_file = File::open("hello.txt")?;
    let mut username = String::new();
    username_file.read_to_string(&mut username)?;
    Ok(username)
}
```

`?`可以串联使用, 上面代码简化为:

```rust
fn read_username_from_file() -> Result<String, io::Error> {
    let mut username = String::new();
    File::open("hello.txt")?.read_to_string(&mut username)?;
    Ok(username)
}
```


## `Trait`

`trait`和`interface`的区别.


### trait bounds

```rust
fn some_function<T: Display + Clone, U: Clone + Debug>(t: &T, u: &U) -> i32 {
```

可以使用`where`改写成: 

```rust
fn some_function<T, U>(t: &T, u: &U) -> i32
    where T: Display + Clone,
          U: Clone + Debug
{
```

## Borrow Checker

`borrow checker`通过对比`borrow`的作用域(scope)来检查borrow是否有效.

```rust
fn main() {
    let r;                // ---------+-- 'a
                          //          |
    {                     //          |
        let x = 5;        // -+-- 'b  |
        r = &x;           //  |       |
    }                     // -+       |
                          //          |
    println!("r: {}", r); //          |
}                         // ---------+
```
Listing 10-17: Annotations of the lifetimes of r and x, named 'a and 'b, respectively