Parsing issue in complex nested match statement with &&.

[StephanvanSchaik]

I think I found a case where I am hitting an issue with Rust's parser which seems to get confused on the following bit of code:

fn main() {
    let x = 2;
    let y = 1;
    
    let test = match x {
        2 => match y {
            1 => true,
            _ => false,
        } && true,
        _ => false,
    };
}

I expected that it would compile similar to the snippet below:

fn main() {
    let x = 2;
    let y = 1;
    
    let test = match x {
        2 => {
            let foo = match y {
                1 => true,
                _ => false,
            };
            foo && true
        }
        _ => false,
    };
}

Instead, it seems to hit a parsing issue following the && after the inner match yielding a bool, where it shows the following error instead:

error: unexpected , in pattern
  --> src/main.rs:11:18
   |
11 |         } && true,
   |                  ^
   |
help: try adding parentheses to match on a tuple...
   |
11 ~         } (&& true,
12 ~         _) => false,
   |
help: ...or a vertical bar to match on multiple alternatives
   |
11 ~         } && true |
12 ~         _ => false,

This happens on:

$ rustc --version
rustc 1.65.0 (897e37553 2022-11-02)

As well as on:

$ rustc +nightly --version
rustc 1.67.0-nightly (73c9eaf21 2022-11-07)

[fmease]

It's not a parser bug. A match expression MatchExpression is classified as an ExpressionWithBlock (and notably not as an ExpressionWithoutBlock) (via) meaning the closing curly brace } terminates the expression early and the parser immediately looks for an optional comma (which in this case it doesn't find, that's fine) followed by a pattern (via).

&&true is a valid pattern (a reference to a reference to true). And thus, it complains about the , after true. It expected a => after the pattern (not sure why the error message is so vague here).

Still, this is a diagnostic issue. The suggestions are not quite on point and we could potentially guide the user in the right direction with some added notes.

@rustbot label A-diagnostics A-suggestion-diagnostics D-terse

[StephanvanSchaik]

Oh, I see. Thanks for the clarification!

I agree that better diagnostics would have been super helpful here in that case.

[lukas-code]

This is related to #102171 and #67347.

The compiler should suggest adding parentheses around the ExpressionWithBlock like this:

fn main() {
    let x = 2;
    let y = 1;
    
    let test = match x {
        2 => (match y {
            1 => true,
            _ => false,
        }) && true,
        _ => false,
    };
}