E0123

A pattern written another language's way

ErrorUnsafe fix

Built from the repository at commit a8614dc.

A pattern takes a variant apart by position: Rect(x, y) binds the first field to x and the second to y. It takes no labels, as in Rect(w: x, h: y), and no braces, as in Rust's Rect { w, h }. It takes no type arguments, as in Option<i64>.Some(v): the type of the matched value gives them.

Why

A pattern lists one sub-pattern for each field, in the order the variant declares them, so a label adds nothing. The fix is unsafe: labels written out of order would swap the bindings when deleted. The fix for type arguments is safe: it deletes them, and the type of the matched value then gives them.

Broken

enum Shape {
    Rect(w: i64, h: i64)
}

fn area(s: Shape) -> i64 {
    match s {
        Rect(w: x, h: y) => x * y
    }
}

fn main() {
    print(area(Shape.Rect(w: 2, h: 3)))
}

What sisu says

error[E0123]: a pattern binds by position: `Rect(x, y)`
 --> E0123.sisu:7:14
  |
7 |         Rect(w: x, h: y) => x * y
  |              ^
  |
help: remove the labels
  |
7 -         Rect(w: x, h: y) => x * y
7 +         Rect(x, y) => x * y
  |

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0123

Fixed

enum Shape {
    Rect(w: i64, h: i64)
}

fn area(s: Shape) -> i64 {
    match s {
        Rect(x, y) => x * y
    }
}

fn main() {
    print(area(Shape.Rect(w: 2, h: 3)))
}