The thing Nim tries to warn me about when using closures

00_Closure_Crash.nim

proc b(x: ptr seq[int]): auto =
  proc a(): seq[int] =
    x[].add(10)
    return x[]
  a

proc ax(): auto =
  # allocate a sequence manually
  # will be GC'd at the first opportunity
  var k = @[1, 2, 3]
  # pass its pointer to the crasherinator
  let m = b(k.addr)
  echo m()
  echo m()
  echo m()
  echo m()
  echo m()
  echo m()
  m
  # k is deleted here, but we return a
  # closure that still holds its reference

proc ll(): auto =
  let p = ax()
  # k is deleted, but the closure env still
  # holds a reference to it
  echo p()
  echo p()
  echo p()
  echo p()
  echo p()
  p

let k = ll()
# boom
echo k()
echo k()
echo k()
echo k()
echo k()
echo k()

01_Closure_Fixed.nim

proc a(x: ref seq[int]): auto =
  # nim knows this ref is still used
  # when this is invoked, and so is
  # still valid (i think)
  proc b(): seq[int] =
    x[].add(10)
    return x[]
  b

proc ax(): auto =
  # create an intentional reference to a sequence
  # will be GC'd properly
  let k = new seq[int]
  # populate this sequence manually
  k[].add(1)
  k[].add(2)
  k[].add(3)
  # before passing it to the crasherinator
  let m = a(k)
  echo m()
  echo m()
  echo m()
  echo m()
  echo m()
  echo m()
  m
  # a reference to k still exists in the closure
  # won't be deleted yet because it's a ref

proc ll(): auto =
  let p = ax()
  echo p()
  echo p()
  echo p()
  echo p()
  echo p()
  p

let k = ll()
echo k()
echo k()
echo k()
echo k()
echo k()
echo k()

# no crash here!