GALA-E0113 — an `inout` place escapes its call

When it fires. An inout parameter lends the caller’s place for one call, so nothing may keep a reference to it once the call returns. The error fires when a function with an inout binding c:

takes &c (or &c.f) for anything but an inout argument: binds it (val p = &c), returns it, or passes it to a parameter that is not inout (a Ptr[T] or *T parameter, a Go function, Println);
captures c in a lambda, a partial function or a by-name argument;
takes a method value of a pointer-receiver method on it (c.inc without a call);
calls a pointer-receiver method on it, or on a field of it, that may keep its receiver: a Ptr-receiver method of this package, a Go pointer method that is not on the compiler’s no-retain allow-list, or a method of another package whose ownership summary does not show its receiver as borrowed. A method with an inout receiver, and a value-receiver method, are always fine.

Minimal repro.

package main

struct Counter(var n int)

func later(inout c Counter) func() int = () => c.n

func main() {
    var c = Counter(1)
    Println(later(&c)())
}

Error output.

error[GALA-E0113]: inout place c is captured by a lambda, which could outlive the call that lends it
  --> main.gala:5:48
  |
5 | func later(inout c Counter) func() int = () => c.n
  |                                                ^ pass the value instead
  |
  = hint: pass the value instead (a copy), or give the lambda its own inout parameter and lend c to it with `&c`

Fix. Read the value while the place is lent, and let the lambda keep the copy:

package main

struct Counter(var n int)

func later(inout c Counter) func() int {
    val n = c.n
    () => n
}

func main() {
    var c = Counter(1)
    Println(later(&c)()) // 1
}

To call a Ptr-receiver method of the same package on an inout place, declare its receiver inout instead.

Rationale. An inout place is borrowed, not owned. A reference that outlives the call would be a second path to the caller’s value, so neither exclusivity nor the translation of the function to value semantics would hold.

GALA-E0113 — an inout place escapes its call

Related

GALA-E0113 — an `inout` place escapes its call