爲什麼在PartialEq/Eq正常工作時匹配不起作用？

Question

我有這個簡單的代碼：

#[derive(Copy, Clone, Default, PartialEq, PartialOrd, Eq, Ord, Hash)] 
struct NodeIndex(u32); 

fn main() { 
    let i = NodeIndex(5323); 
    let from = NodeIndex(21030); 
    let to = NodeIndex(21031); 

    println!("from == i => {}, to == i => {}", from == i, to == i); 

    match i { 
     from => println!("1"), 
     to => println!("2"), 
     _ => println!("other"), 
    } 
} 

它打印：

from == i => false, to == i => false 
1 

所以i != from，i != to，但match通話from => println!("1"),， 是怎麼回事？

Answer 1

可能更容易理解發生了什麼事情通過查看編譯器的警告：

warning: unused variable: `from` 
    --> src/main.rs:12:9 
    | 
12 |   from => println!("1"), 
    |   ^^^^ 
    | 
    = note: #[warn(unused_variables)] on by default 

warning: unreachable pattern 
    --> src/main.rs:13:9 
    | 
13 |   to => println!("2"), 
    |   ^^ this is an unreachable pattern 
    | 
    = note: #[warn(unreachable_patterns)] on by default 
note: this pattern matches any value 
    --> src/main.rs:12:9 
    | 
12 |   from => println!("1"), 
    |   ^^^^ 

warning: unused variable: `to` 
    --> src/main.rs:13:9 
    | 
13 |   to => println!("2"), 
    |   ^^ 
    | 
    = note: #[warn(unused_variables)] on by default 

warning: unreachable pattern 
    --> src/main.rs:14:9 
    | 
14 |   _ => println!("other"), 
    |  ^this is an unreachable pattern 
    | 
    = note: #[warn(unreachable_patterns)] on by default 
note: this pattern matches any value 
    --> src/main.rs:12:9 
    | 
12 |   from => println!("1"), 
    |   ^^^^ 

基本上標識符from和to不是指包含在綁定的值。它們是新的恰巧匹配任何東西的綁定。同樣的事情發生了新的標識符名稱：

match i { 
    x => println!("1"), 
    y => println!("2"), 
    _ => println!("other"), 
} 

由於程序總是第一個匹配的情況下，始終印有「1」。

可以通過聲明from和to爲const得到預期的結果：

const from: NodeIndex = NodeIndex(21030); 
const to: NodeIndex = NodeIndex(21031); 

，或者直接使用對象文本：

match i { 
    NodeIndex(21030) => println!("1"), 
    NodeIndex(21031) => println!("2"), 
    _ => println!("other"), 
} 

如果from和to值中唯一已知運行時，您可以使用if/else語句：

比賽（在運行時計算）時項

if n == from { 
    println!("1"); 
} else if n == to { 
    println!("2"); 
} else { 
    println!("other"); 
} 

...或添加if條款：

match i { 
    n if n == from => println!("1"), 
    n if n == to => println!("2"), 
    _ => println!("other"), 
}