message: Increase max stack size to 50
  Summary: Increase starlark stack size from 40 to 50.
diff: @@ -191,7 +191,7 @@ impl<'v> Default for CheapCallStack<'v> {
  // [1] https://docs.rs/tokio/0.2.1/tokio/runtime/struct.Builder.html#method.thread_stack_size
  // TODO(nga): count loops in call stack size.
  // TODO(nga): make it configurable.
  -const MAX_CALLSTACK_RECURSION: usize = 40;
  +const MAX_CALLSTACK_RECURSION: usize = 50;
  unsafe impl<'v> Trace<'v> for CheapCallStack<'v> {
  fn trace(&mut self, tracer: &Tracer<'v>) {
  