Assignment to an immutable binding

Meaning

Direct assignment or compound assignment targets a binding not declared mut.

Bindings are immutable by default so state changes remain visible at the declaration. This diagnostic concerns reassignment, not whether a move-only value is still owned.

Why it happens

An accumulator, loop counter, or state binding was declared without mut, or a refactor changed an expression into in-place assignment.

Invalid code

fn main() -> Int {
    let value = 5
    value += 1
    return value
}

Correct code

fn main() -> Int {
    let mut value = 5
    value += 1
    return value
}

Common fixes

Add mut when mutation is intentional, or compute a new binding instead of reassigning.

Do not add mut automatically to silence unrelated ownership errors. It does not permit use after move. Struct field assignment has a documented 0.1 edge case that is currently less strict than direct binding assignment; do not depend on it as the future mutability model.