Cannot use parens for grouping in patterns

[huonw]

fn main() {
    let &mut _ok = &1;
    let &(mut _error) = &1;
}

paren-pat.rs:3:20: 3:21 error: expected `,` but found `)`
paren-pat.rs:3     let &(mut _error) = &1;
                                   ^

This is a very minor papercut for when one wish to clarify exactly what a pattern is doing; the example above is the main situation in which I've wanted parens, making it clear that it's a & pattern + a mutable ident binding, rather than a (hypothetical) &mut pattern.

Closing this as a dupe of #19580.

[ftxqxd]

I don’t really understand how this was closed as a dupe rather than #19580. How can this be a dupe of a more recent and less general issue? And in any case, I think that this issue is probably RFC-like enough to belong in the RFCs repo.