GALA-E0112 — one call lends overlapping `inout` places

When it fires. A call lends each inout place exclusively: for the duration of the call, nothing else reads or writes it. The error fires when one call lends the same place twice, or a place and a field of it:

two inout arguments name overlapping places: both(&c, &c), f(&p, &p.lex);
a method with an inout receiver gets an inout argument that overlaps the receiver: p.settle(&p.left);
a lambda argument of the call reads a place the call lends: withFn(&c, () => c.n).

Disjoint fields of one place may be lent to one call: both(&p.left, &p.right).

Minimal repro.

package main

struct Counter(var n int)

func both(inout a Counter, inout b Counter) {
    a.n = a.n + b.n
}

func main() {
    var c = Counter(1)
    both(&c, &c)
    Println(c.n)
}

Error output.

error[GALA-E0112]: &c and &c overlap: a call cannot lend one place twice
  --> main.gala:11:14
   |
11 |     both(&c, &c)
   |              ^^ lend disjoint places, or copy one of them into a local `var`…
   |
   = hint: lend disjoint places, or copy one of them into a local `var` first

Fix. Lend disjoint places. To pass the same value twice, copy it into a local var first:

package main

struct Counter(var n int)

func both(inout a Counter, inout b Counter) {
    a.n = a.n + b.n
}

func main() {
    var c = Counter(1)
    var other = c
    both(&c, &other)
    Println(c.n) // 2
}

Rationale. The callee may assume that its inout parameters are distinct places, as Swift’s law of exclusivity guarantees. With two paths to one place, a write through one is seen through the other, which neither the callee nor its translation to value semantics expects.

GALA-E0112 — one call lends overlapping inout places

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GALA-E0112 — one call lends overlapping `inout` places