How to reference variables in destroy-time provisioner in Terraform?

Are you using a local-exec provisioner to run upon destruction with a clause when = destroy and getting this error message:

Destroy-time provisioners and their connection configurations may only reference attributes of the related resource, via ‘self’, ‘count.index’, or ‘each.key’.

A workaround is to use triggers, so your code would like this:

resource "null_pointer" {
  triggers = {
    local_var = var.external_var
  }

  provisioner "local-exec" {
    when = destroy
    command = "echo $self.triggers.local_var"
  }
}

Instead of this:

resource "null_pointer" {
  provisioner "local-exec" {
    when = destroy
    command = "echo $var.external_var"
  }
}