What does the ‘@’ symbol do in Rust?

You can use the @ symbol to bind a pattern to a name. As the Rust Reference demonstrates:

let x = 1;

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

Assignments in Rust allow pattern expressions (provided they are complete) and argument lists are no exception. In the specific case of @, this isn’t very useful because you can already name the matched parameter. However, for completeness, here is an example which compiles:

enum MyEnum {
    TheOnlyCase(u8),
}

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