Cannot use parens for grouping in patterns

[emberian]

The fact that

fn main() {
    let x = &4i;

    match x {
        // wanted to type `e @ &1 ... &4`
        e @ &1 ... 4 => println!("Got a small value: {}", e),
        &val => println!("Got a value: {}", val),
    }
}

works but

fn main() {
    let x = &4i;

    match x {
        // wanted to type `e @ &1 ... &4`
        e @ &(1 ... 4) => println!("Got a small value: {}", e),
        &val => println!("Got a value: {}", val),
    }
}

is quite surprising. Presumably the &( is expecting a tuple pattern, should consider investigating and seeing if patterns can be parenthesized in general.

cc #11144

[ghost]

Needs an RFC.

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#554