解構對

Question

以下兩行的盒時Collat​​erally移動誤差

error[E0382]: use of moved value: `x` 
--> src/main.rs:3:13 
    | 
3 |  let (a, b) = *x; 
    |   -^value used here after move 
    |   | 
    |   value moved here 
    | 
    = note: move occurs because `x.0` has type `std::string::String`, which does not implement the `Copy` trait 

有趣的是，如果我這樣做與枚舉類型與多個零件 ，我得到一個稍微不同的錯誤：

enum Tree { 
    Nil, 
    Pair(Box<Tree>, Box<Tree>), 
} 

fn main() { 
    let x = Box::new(Tree::Nil); 

    match *x { 
     Tree::Pair(a, b) => Tree::Pair(a, b), 
     _ => Tree::Nil, 
    }; 
} 

我得到th E錯誤：

error[E0382]: use of collaterally moved value: `(x:Tree::Pair).1` 
    --> src/main.rs:10:23 
    | 
10 |   Tree::Pair(a, b) => Tree::Pair(a, b), 
    |     -^value used here after move 
    |     | 
    |     value moved here 
    | 
    = note: move occurs because `(x:Tree::Pair).0` has type `std::boxed::Box<Tree>`, which does not implement the `Copy` trait 

爲什麼會發生這種情況，我怎麼能輕易破壞結構與let/match，讓內部零件的所有權？我知道我可以首先解引用並命名結構，但是如果我將模式匹配深入到結構中，則會變得非常冗長。

Answer 1

你偶然發現了limitation on destructuring and boxes。幸運的是，解決這些問題很容易。所有你需要做的是推出一個包含整個結構一個新的中介變量，解構來自：

let x = Box::new(("slefj".to_string(), "a".to_string())); 
let pair = *x; 
let (a, b) = pair; 

第二個例子：

let pair = *x; 
match pair { 
    Tree::Pair(a, b) => Tree::Pair(a, b), 
    _ => Tree::Nil, 
};