match do catch-22

[durka]

It seems that you need to add parentheses in order to put a catch expression in a match head, but if you do, you get a warning:

#![feature(catch_expr)]

fn main() {
    match (do catch { }) { } //~WARN unnecessary parentheses around `match` head expression
    match do catch { } { } //~ERROR expected expression, found reserved keyword `do`
}

Why are the parens required anyway?

[kennytm]

I think the original idea is to make this unambiguous? It was even checked in b1aa993.

let catch = 5;
match catch { ...

[durka]

catch is already ambiguous with struct init syntax, I thought that's why we added do.

[shepmaster]

catch is already ambiguous with struct init syntax,

But struct init syntax is disallowed in a match head.

[durka]

Then there's no ambiguity anyway. I don't understand the point of banning this. Maybe it's just been thrown in the large bin of "deal with this once we figure out how to get rid of do".

[durka]

This somehow got even worse: now match (do catch {}) {} errors saying the new syntax is try, but obviously that doesn't compile either without enabling the new edition.

(And in E2018, match (try {}) {} still has a warning telling you remove parentheses that you can't remove.)

[kennytm]

Given #54885 I'm labeling this a diagnostic problem.

[Mark-Simulacrum]

Updated code, still lints on the unused parens but errors without them.

#![feature(try_blocks)]

fn main() {
    match (try { Ok::<_, ()>(0) }) {
        Ok(_) => {},
        Err(()) => {}
    }
}

[bjorn3]

This now compiles:

#![feature(try_blocks)]

fn main() {
    match try { Ok::<_, ()>(0) } {
        Ok(_) => {},
        Err(()) => {}
    }
}