LibJS: Allow cells to mark null pointers

This simplifies the cell visiting functions by letting them not worry
about the pointers they pass to the visitor being null.

Libraries/LibJS/Heap/Heap.cpp

@@ -188,7 +188,7 @@ class MarkingVisitor final : public Cell::Visitor {
 public:
     MarkingVisitor() {}
 
-    virtual void visit(Cell* cell)
+    virtual void visit_impl(Cell* cell)
     {
         if (cell->is_marked())
             return;
@@ -206,11 +206,8 @@ void Heap::mark_live_cells(const HashTable<Cell*>& roots)
     dbg() << "mark_live_cells:";
 #endif
     MarkingVisitor visitor;
-    for (auto* root : roots) {
-        if (!root)
-            continue;
+    for (auto* root : roots)
         visitor.visit(root);
-    }
 }
 
 void Heap::sweep_dead_cells()

Libraries/LibJS/Runtime/Cell.cpp

@@ -34,10 +34,16 @@
 
 namespace JS {
 
+void Cell::Visitor::visit(Cell* cell)
+{
+    if (cell)
+        visit_impl(cell);
+}
+
 void Cell::Visitor::visit(Value value)
 {
     if (value.is_cell())
-        visit(value.as_cell());
+        visit_impl(value.as_cell());
 }
 
 Heap& Cell::heap() const

Libraries/LibJS/Runtime/Cell.h

@@ -49,8 +49,11 @@ public:
 
     class Visitor {
     public:
-        virtual void visit(Cell*) = 0;
+        void visit(Cell*);
         void visit(Value);
+
+    protected:
+        virtual void visit_impl(Cell*) = 0;
     };
 
     virtual void visit_children(Visitor&) {}

Libraries/LibJS/Runtime/LexicalEnvironment.cpp

@@ -45,7 +45,6 @@ LexicalEnvironment::~LexicalEnvironment()
 void LexicalEnvironment::visit_children(Visitor& visitor)
 {
     Cell::visit_children(visitor);
-    if (m_parent)
     visitor.visit(m_parent);
     for (auto& it : m_variables)
         visitor.visit(it.value.value);

Libraries/LibJS/Runtime/Shape.cpp

@@ -81,9 +81,7 @@ Shape::~Shape()
 void Shape::visit_children(Cell::Visitor& visitor)
 {
     Cell::visit_children(visitor);
-    if (m_prototype)
     visitor.visit(m_prototype);
-    if (m_previous)
     visitor.visit(m_previous);
     for (auto& it : m_forward_transitions)
         visitor.visit(it.value);