Improving error for unnecessary parentheses in `for` loop head

[badboy]

The following code:

fn main() {
    let v = vec![1,2,3];
    
    for (x in v) { }
}

leads to this error:

error: expected one of `)`, `,`, or `@`, found `in`
 --> src/main.rs:4:12
  |
4 |     for (x in v) { }
  |            ^^ expected one of `)`, `,`, or `@` here

(playground)

A colleague of mine typed this code, coming with a C++/JavaScript/Kotlin background where parentheses are required in the for loop head.

I wonder if it’s possible/worth it to make that error message nicer and easier to understand. (@estebank asked me to file this issue, so it doesn't get lost)

[hellow554]

I wonder why this isn't accepted. You can write if (x == 3) { }, so shouldn't this code be accepted as well? 🤔

[badboy]

I wonder why this isn't accepted. You can write if (x == 3) { }, so shouldn't this code be accepted as well? 🤔

The syntax for that loop is explicitly "for" $pattern "in" $expression {}.
For if the syntax would be if $expression {} and only the expression can contain the parentheses.
Of course it could be just accepted with optional parentheses for the loop, but we don't really want that (and parentheses in if are a lint-warning)