Printer: Make prefix() [[nodiscard]] and fix bug printing Dummy nodes

Dummy and DummyAllocator nodes had been changed to use NodeIds, but
were still printed out in full when visited for a second time.

[[nodiscard]] prevents future bugs of this type by turning them into
compilation errors.

Example of the now-fixed bug:
    [1]  Container: std::map<int32_t, int32_t, DummySizedOperator<0, 0, 8>, std::allocator<std::pair<int32_t const, int32_t>>>
            Param
              Primitive: int32_t
            Param
              Primitive: int32_t
            Param
    [2]       Dummy [less<int>]
            Param
              ...
    [3]   Container: std::map<int32_t, int32_t, DummySizedOperator<0, 0, 8>, std::allocator<std::pair<int32_t const, int32_t>>>
            Param
              Primitive: int32_t
            Param
              Primitive: int32_t
            Param
              [2]
    Dummy [less<int>]
            Param
              ...

With this patch, the second "Dummy" line will not be printed.

oi/type_graph/Printer.cpp

@@ -37,7 +37,7 @@ void Printer::print(const Type& type) {
 }
 
 void Printer::visit(const Class& c) {
-  if (prefix(&c))
+  if (prefix(c))
     return;
 
   std::string kind;
@@ -79,7 +79,7 @@ void Printer::visit(const Class& c) {
 }
 
 void Printer::visit(const Container& c) {
-  if (prefix(&c))
+  if (prefix(c))
     return;
 
   out_ << "Container: " << c.name() << " (size: " << c.size() << ")"
@@ -106,7 +106,7 @@ void Printer::visit(const Enum& e) {
 }
 
 void Printer::visit(const Array& a) {
-  if (prefix(&a))
+  if (prefix(a))
     return;
 
   out_ << "Array: ";
@@ -117,7 +117,7 @@ void Printer::visit(const Array& a) {
 }
 
 void Printer::visit(const Typedef& td) {
-  if (prefix(&td))
+  if (prefix(td))
     return;
 
   auto name = td.name();
@@ -129,7 +129,7 @@ void Printer::visit(const Typedef& td) {
 }
 
 void Printer::visit(const Pointer& p) {
-  if (prefix(&p))
+  if (prefix(p))
     return;
 
   out_ << "Pointer";
@@ -140,36 +140,41 @@ void Printer::visit(const Pointer& p) {
 }
 
 void Printer::visit(const Dummy& d) {
-  prefix(&d);
+  if (prefix(d))
+    return;
   out_ << "Dummy ";
   out_ << "[" << d.inputName() << "] ";
   out_ << "(size: " << d.size() << align_str(d.align()) << ")" << std::endl;
 }
 
 void Printer::visit(const DummyAllocator& d) {
-  prefix(&d);
+  if (prefix(d))
+    return;
   out_ << "DummyAllocator ";
   out_ << "[" << d.inputName() << "] ";
   out_ << "(size: " << d.size() << align_str(d.align()) << ")" << std::endl;
   print(d.allocType());
 }
 
-bool Printer::prefix(const Type* type) {
+void Printer::prefix() {
+  int indent = baseIndent_ + depth_ * 2;
+  out_ << std::string(indent, ' ');
+}
+
+bool Printer::prefix(const Type& type) {
   int indent = baseIndent_ + depth_ * 2;
 
-  if (type) {
+  if (tracker_.visit(type)) {
-    if (tracker_.visit(*type)) {
+    // Node has already been printed - print a reference to it this time
-      // Node has already been printed - print a reference to it this time
+    out_ << std::string(indent, ' ');
-      out_ << std::string(indent, ' ');
+    out_ << "[" << type.id() << "]" << std::endl;
-      out_ << "[" << type->id() << "]" << std::endl;
+    return true;
-      return true;
-    }
-
-    std::string nodeId = "[" + std::to_string(type->id()) + "]";
-    out_ << nodeId;
-    indent -= nodeId.size();
   }
 
+  std::string nodeId = "[" + std::to_string(type.id()) + "]";
+  out_ << nodeId;
+  indent -= nodeId.size();
+
   out_ << std::string(indent, ' ');
   return false;
 }

oi/type_graph/Printer.h

@@ -43,7 +43,8 @@ class Printer : public ConstVisitor {
   void visit(const DummyAllocator& d) override;
 
  private:
-  bool prefix(const Type* type = nullptr);
+  void prefix();
+  [[nodiscard]] bool prefix(const Type& type);
   void print_param(const TemplateParam& param);
   void print_parent(const Parent& parent);
   void print_member(const Member& member);