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GALA-E0111 — inout argument is not & of a mutable place

When it fires. An inout parameter lends the caller’s place to the callee for the call (see inout parameters). Its argument must say so: it is &place, where the place is

The error fires when the & is missing (bump(c)), or when the operand of & is not a mutable place: a call result (&mk()) or a literal. (A val or a val field is already GALA-E0076.) It also fires on an inout parameter declared variadic or with a default, and on a struct field declared inout: none of them would lend one place of the caller.

Minimal repro.

package main

struct Counter(var n int)

func bump(inout c Counter) {
    c.n = c.n + 1
}

func main() {
    var c = Counter(0)
    bump(c)
    Println(c.n)
}

Error output.

error[GALA-E0111]: the argument c of bump is passed to an inout parameter without `&`
  --> main.gala:11:10
   |
11 |     bump(c)
   |          ^ lend the place with `&c`
   |
   = hint: lend the place with `&c`; an inout argument is marked at the call site

Fix. Mark the argument with &. The call site then shows that c may change:

package main

struct Counter(var n int)

func bump(inout c Counter) {
    c.n = c.n + 1
}

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

To lend a value that is not a place, copy it into a var first: var tmp = mk(), then bump(&tmp).

Rationale. & at the call site is Swift’s rule for inout: a reader sees which arguments a call may change. It is also the Go that is generated: an inout x T is the Go parameter x *T.