What does the '@' symbol do in Rust?

Question

I forgot to specify the type of a parameter and the error message was as follows:

error: expected one of `:` or `@`, found `)`
 --> src/main.rs:2:12
  |
2 | fn func(arg)
  |            ^ expected one of `:` or `@` here

Which raises the question: what can you do with an @ symbol? I don't remember reading about using the @ symbol for anything. I also did some Googling and couldn't find anything. What does @ do?

Answer 1

您可以使用@符号将模式绑定到名称。正如Rust Reference demonstrates：

let x = 1;

match x {
    e @ 1 ... 5 => println!("got a range element {}", e),
    _ => println!("anything"),
}

Rust allow pattern expressions中的分配（前提是它们已完成）和argument lists are no exception。在@的特定情况下，这不是非常有用，因为您已经可以命名匹配的参数。但是，为了完整性，这里有一个编译的例子：

enum MyEnum {
    TheOnlyCase(u8),
}

fn my_fn(x @ MyEnum::TheOnlyCase(_): MyEnum) {}