retire TrailedAttrVarHeapLink TrailEntry tag

src/machine/machine_state_impl.rs

@@ -130,16 +130,6 @@ impl MachineState {
                     }
                 }
             }
-            TrailRef::AttrVarHeapLink(h) => {
-                if h < self.hb {
-                    self.trail.push(TrailEntry::build_with(
-                        TrailEntryTag::TrailedAttrVarHeapLink,
-                        h as u64,
-                    ));
-
-                    self.tr += 1;
-                }
-            }
             TrailRef::AttrVarListLink(h, l) => {
                 if h < self.hb {
                     self.trail.push(TrailEntry::build_with(

src/machine/mod.rs

@@ -863,9 +863,6 @@ impl Machine {
                 TrailEntryTag::TrailedAttrVar => {
                     self.machine_st.heap[h] = attr_var_as_cell!(h);
                 }
-                TrailEntryTag::TrailedAttrVarHeapLink => {
-                    self.machine_st.heap[h] = heap_loc_as_cell!(h);
-                }
                 TrailEntryTag::TrailedAttrVarListLink => {
                     let l = self.machine_st.trail[i + 1].get_value() as usize;
 

src/types.rs

@@ -53,7 +53,6 @@ pub enum HeapCellValueView {
     // trail elements.
     TrailedHeapVar = 0b011101,
     TrailedStackVar = 0b011111,
-    TrailedAttrVarHeapLink =  0b100001,
     TrailedAttrVarListLink =  0b100011,
     TrailedAttachedValue =    0b100101,
     TrailedBlackboardEntry =  0b100111,
@@ -182,7 +181,6 @@ impl Ref {
 #[derive(Debug, Clone, Copy)]
 pub enum TrailRef {
     Ref(Ref),
-    AttrVarHeapLink(usize),
     AttrVarListLink(usize, usize),
     BlackboardEntry(Atom),
     BlackboardOffset(Atom, HeapCellValue), // key atom, key value
@@ -194,7 +192,6 @@ pub(crate) enum TrailEntryTag {
     TrailedHeapVar = 0b011110,
     TrailedStackVar = 0b011111,
     TrailedAttrVar = 0b101110,
-    TrailedAttrVarHeapLink = 0b100010,
     TrailedAttrVarListLink = 0b100011,
     TrailedAttachedValue = 0b101010,
     TrailedBlackboardEntry = 0b100110,