Destructuring assignment only supports `=`

Why this fires¶

A destructuring assignment target — a tuple (a, b) = ... or a struct Struct { x, y } = ... — was used with something other than plain =, or had an invalid slot/field target. Compound assignment operators (+=, -=, ...) have no per-element meaning when the left-hand side is a whole pattern, and .. spread makes no sense as an assignment target (there's nothing to name).

fn f() {
    var a = 1
    var b = 2;
    (a, b) += (1, 2)
    //      ^^ error[TE130]: destructuring assignment only supports `=`
    //                        (compound ops have no per-element meaning)
}

(The semicolon after var b = 2 matters here — without it, (a, b) on the next line parses as a call continuation of the previous statement instead of a new tuple-assignment target.)

struct P { x: int, y: int }

fn f(p: P) {
    var x = 0
    P { x, ..r } = p
    //     ^^^ error[TE130]: `..` is not allowed in an assignment target —
    //         name the fields you assign
}

Fix it¶

1. Use plain =¶

fn f() {
    var a = 1
    var b = 2
    (a, b) = (1, 2)          // ok
}

2. Don't spread into an assignment target¶

struct P { x: int, y: int }

fn f(p: P) {
    var x = 0
    var y = 0
    P { x, y } = p           // ok — name every field you assign
}

See also¶

• TE129 — tuple-assignment value-count mismatch.
• TE005 — type mismatch in a destructuring-assignment slot.
• /docs/pattern-matching — destructuring assignment.