GALA-E0076 — Address of an immutable value

When it fires. &x takes the address of something that cannot change:

a val, local or package-level;
a function, method or lambda parameter not declared var;
a receiver;
an immutable field (n in struct Obj(n int));
a field reached through a val (&o.n where o is a val).

&x is a *T, and a *T can write, so a pointer to any of these would be a way to change it. Only something that can change has an address: a var, a var field of a var, a field of a pointee, a composite literal.

Minimal repro.

package main

struct Counter(var N int)

func bump(c *Counter) {
    c.N = c.N + 1
}

func main() {
    val c = Counter(0)
    bump(&c)
    Println(c.N)
}

Error output.

error[GALA-E0076]: cannot take the address of c: c is a val
  --> main.gala:11:10
   |
11 |     bump(&c)
   |          ^^ pass c itself
   |
   = hint: pass c itself; declare it `var c` where a pointer is needed

Fix. A function that only reads takes the value:

package main

struct Counter(var N int)

func show(c Counter) string = s"count ${c.N}"

func main() {
    val c = Counter(0)
    Println(show(c))
}

A function that changes it, or a Go API that wants a *T, gets the address of a var:

package main

struct Counter(var N int)

func bump(c *Counter) {
    c.N = c.N + 1
}

func main() {
    var c = Counter(0)
    bump(&c)
    Println(c.N) // 1
}

For a parameter, declare it var n. For a receiver, declare the method on a pointer receiver (func (c *Counter) ...), which is already a pointer. For an immutable field, declare the field var.

Rationale. A val, a parameter, a receiver and an immutable field are immutable, and a pointer is the one way to change a value without naming it. Earlier versions gave &x of such a value a read-only ConstPtr[T], which no Go API accepted where a *T was expected, copied the value on every field read and could be nil. A value that is only read is passed itself: it is already immutable.