config: detect provisioner-only resource in JSON and error [GH-4385]

config/loader_hcl.go

@@ -405,6 +405,15 @@ func loadResourcesHcl(list *ast.ObjectList) ([]*Resource, error) {
  // Now go over all the types and their children in order to get
  // all of the actual resources.
  for _, item := range list.Items {
+ // GH-4385: We detect a pure provisioner resource and give the user
+ // an error about how to do it cleanly.
+ if len(item.Keys) == 4 && item.Keys[2].Token.Value().(string) == "provisioner" {
+ return nil, fmt.Errorf(
+ "position %s: provisioners in a resource should be wrapped in a list\n\n"+
+ "Example: \"provisioner\": [ { \"local-exec\": ... } ]",
+ item.Pos())
+ }
+
  if len(item.Keys) != 2 {
  return nil, fmt.Errorf(
  "position %s: resource must be followed by exactly two strings, a type and a name",