W8001 copy_on_write

A warning from the package phase. sakura explain W8001 prints this explanation offline.

An update to a shared collection is forced to copy inside a loop because another reference retains the old value (design §8.2). Drop the retaining use or restructure the loop. The code is registered ahead of its check, which comes with the native backend's reuse analysis (M5); no command reports it yet.

Wrong programCopy Anchor Link

compiler/tests/ui/W8001_copy_on_write.sakura, which the compiler's ui suite does not run yet (needs: native backend with Perceus reuse analysis):

fn grow() -> List<Int> {
  var xs = [1]
  var kept = xs
  for i in 0..100 {
    kept = xs
    xs = List.push(xs, i)
  }
  xs
}

Fixed programCopy Anchor Link

No fixed program is checked in for this code yet; the explanation above describes the repair.