correct depth calculation for lists that are their own car (#1876)

src/heap_print.rs

@@ -861,7 +861,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
  )
  }
 
- fn check_for_seen(&mut self, max_depth: usize) -> Option<HeapCellValue> {
+ fn check_for_seen(&mut self, max_depth: &mut usize) -> Option<HeapCellValue> {
  if let Some(mut orig_cell) = self.iter.next() {
  loop {
  let is_cyclic = orig_cell.get_forwarding_bit();
@@ -899,14 +899,26 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
  });
  }
  None => {
- if self.max_depth == 0 || max_depth == 0 {
+ if self.max_depth == 0 || *max_depth == 0 {
  // otherwise, contract it to an ellipsis.
  push_space_if_amb!(self, "...", {
  append_str!(self, "...");
  });
  } else {
  debug_assert!(cell.is_ref());
 
+ // as usual, the WAM's
+ // optimization of the Lis tag
+ // (conflating the location of
+ // the list and that of its
+ // first element) needs
+ // special consideration here
+ // lest we find ourselves in
+ // an infinite loop.
+ if cell.get_tag() == HeapCellValueTag::Lis {
+ *max_depth -= 1;
+ }
+
  let h = cell.get_value() as usize;
  self.iter.push_stack(IterStackLoc::iterable_loc(h, HeapOrStackTag::Heap));
 
@@ -1363,7 +1375,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
 
  self.state_stack.push(TokenOrRedirect::FunctorRedirect(max_depth));
  self.state_stack.push(TokenOrRedirect::HeadTailSeparator); // bar
- self.state_stack.push(TokenOrRedirect::FunctorRedirect(max_depth));
+ self.state_stack.push(TokenOrRedirect::FunctorRedirect(max_depth + 1));
 
  self.open_list(switch);
  }
@@ -1563,10 +1575,15 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
  &mut self,
  op: Option<DirectedOp>,
  is_functor_redirect: bool,
- max_depth: usize,
+ mut max_depth: usize,
  ) {
  let negated_operand = negated_op_needs_bracketing(&self.iter, self.op_dir, &op);
 
+ let addr = match self.check_for_seen(&mut max_depth) {
+ Some(addr) => addr,
+ None => return,
+ };
+
  let print_struct = |printer: &mut Self, name: Atom, arity: usize| {
  if name == atom!("[]") && arity == 0 {
  match printer.state_stack.last() {
@@ -1628,11 +1645,6 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter> {
  }
  };
 
- let addr = match self.check_for_seen(max_depth) {
- Some(addr) => addr,
- None => return,
- };
-
  if !addr.is_var()
  && !addr.is_compound(&self.iter.heap)
  && self.max_depth_exhausted(max_depth)