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GALA-E0114 — inout method called on a value that is not a mutable place

When it fires. A method with an inout receiver, func (inout c T) m(), lends the receiver’s place for the call. It is called on a mutable place: a var, an inout parameter, a var field of one, or a Ptr[T]. The error fires when it is called on anything else: a val, a parameter not declared var, a val field, a call result or a literal. It fires even when the method only reads its receiver.

Minimal repro.

package main

struct Counter(var n int)

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

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

Error output.

error[GALA-E0114]: cannot call bump on this Counter: bump has an inout receiver, and this Counter is not a mutable place
  --> main.gala:11:7
   |
11 |     c.bump()
   |       ^^^^ call it on a `var`, an inout parameter or a `Ptr`
   |
   = hint: call it on a `var`, an inout parameter or a `Ptr`; to change a copy, copy the value into a `var` first

Fix. Call it on a place: declare the value var, or copy it into one.

package main

struct Counter(var n int)

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

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

Rationale. A Ptr receiver called on a val runs on a hidden copy when it only reads its receiver, and is GALA-E0110 when it can change it. An inout receiver is a contract that the method borrows a place, so it is never called on a copy. A method that only reads its receiver needs no inout: give it a value receiver, func (c T) m().