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GALA-E0116 — inout interface method implemented without an inout receiver

When it fires. An interface may declare a method inout:

type Stepper interface {
    inout Step() int
}

A value fills such an interface only when its type implements each inout method with an inout receiver, func (inout c Counter) Step() int. The error fires when the method has another receiver (a Ptr, a *T or a value receiver), or when the type is declared in Go. A Go type is accepted only when its method is on the compiler’s no-retain allow-list (bytes.Buffer’s WriteString, for example), whose entries are checked against the Go compiler’s escape analysis.

An interface value that is already an interface, built in GALA or received from Go, fills it as it is: its methods are trusted, not checked.

Minimal repro.

package main

type Stepper interface {
    inout Step() int
}

struct Counter(var n int)

func (c Ptr[Counter]) Step() int {
    c.n = c.n + 1
    c.n
}

func run(s Stepper) int = s.Step()

func main() {
    Println(run(&Counter(0)))
}

Error output.

error[GALA-E0116]: Counter cannot fill Stepper: Stepper declares Step inout, and Counter.Step has no inout receiver
  --> main.gala:17:17
   |
17 |     Println(run(&Counter(0)))
   |                 ^^^^^^^^^^^ declare it `func
   |
   = hint: declare it `func (inout r Counter) Step(...)`

Fix. Declare the method with an inout receiver:

package main

type Stepper interface {
    inout Step() int
}

struct Counter(var n int)

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

func run(s Stepper) int = s.Step()

func main() {
    Println(run(&Counter(0))) // 1
}

To use a Go type, wrap it in a GALA type whose method has an inout receiver.

Rationale. An inout method borrows its receiver for the call: it keeps no reference to it and nothing else reaches it meanwhile. A Ptr-receiver method or a Go method makes no such promise, so code calling the interface’s inout method could not rely on it.