LYR-SEM0019

a write to something that cannot be written

error · spec 03-types

An assignment, ++/--, an index write or a call of a mut fn targets something that is not a writable place: a let, a parameter, a field without var, a temporary, a module let, a value reached through a let or through an optional chain.

Explanation

Only a place can be written: a var local or global, a var field reached through writable places, an element of a writable array. A value type is frozen where it is held by a let, and a mut fn writes its receiver, so it needs a writable one too.

Declare the binding or the field with var, or write to a copy held in a var.

Wrong

fn main(): void {
    let n = 1;
    n = 2;
    let _ = n;
}

Right

fn main(): void {
    var n = 1;
    n = 2;
    let _ = n;
}