rust - ref 能做什么而 references 不能？

Question

ref 可以做什么而引用不能？可以

match value.try_thing() {
    &Some(ref e) => do_stuff(e),
    // ...
}

不等于

match value.try_thing() {
    &Some(e) => do_stuff(&e),
    // ...
}

Answer 1

Editor's note — this answer was posted when the question was titled "Was Rust's ref keyword avoidable?". The OP has since changed the title, making the phrasing of the answer less sensible.

不，您建议的语法无法避免。在允许移动的情况下，您的语法不允许引用。在此示例中，inner 是 val 中整数的副本，更改它对 val 没有影响:

fn main() {
    let mut val = Some(42);
    
    if let &mut Some(mut inner) = &mut val {
        inner += 1;
    }
    
    println!("{:?}", val); // Some(42)
}

需要ref关键字来强制引用:

fn main() {
    let mut val = Some(42);
    
    if let &mut Some(ref mut inner) = &mut val {
        *inner += 1;
    }
    
    println!("{:?}", val); // Some(43)
}

Match ergonomics 允许以更简单的方式编写:

fn main() {
    let mut val = Some(42);
    
    if let Some(inner) = &mut val {
        *inner += 1;
    }
    
    println!("{:?}", val);
}

但是，如果我们只从这种语法开始，那么我们可能会遇到相反的问题和关键字，一个是强制移动；也许 Some(move inner)。在那个替代宇宙中，会有一个问题询问 move 关键字是否可以避免。

另见:

• How can the ref keyword be avoided when pattern matching in a function taking &self or &mut self?
• How does Rust pattern matching determine if the bound variable will be a reference or a value?
• Why is `ref` used instead of an asterisk in pattern matching?
• What is the syntax to match on a reference to an enum?
• Rust by example: The ref pattern