LibJS: Don't use GC::Root unnecessarily in Error stack traces

This was causing a fair bit of root registration churn on pages that
throw lots of errors.

Since there's no need for these pointers to float around freely, we can
just visit them during the mark phase as usual.

Author: awesomekling

Libraries/LibJS/Runtime/Error.cpp

@@ -58,6 +58,13 @@ Error::Error(Object& prototype)
     populate_stack();
 }
 
+void Error::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    for (auto& frame : m_traceback)
+        visitor.visit(frame.cached_source_range);
+}
+
 // 20.5.8.1 InstallErrorCause ( O, options ), https://tc39.es/ecma262/#sec-installerrorcause
 ThrowCompletionOr<void> Error::install_error_cause(Value options)
 {

Libraries/LibJS/Runtime/Error.h

@@ -21,7 +21,7 @@ struct JS_API TracebackFrame {
     Utf16String function_name;
     [[nodiscard]] SourceRange const& source_range() const;
 
-    GC::Root<CachedSourceRange> cached_source_range;
+    GC::Ptr<CachedSourceRange> cached_source_range;
 };
 
 enum CompactTraceback {
@@ -51,6 +51,8 @@ class JS_API Error : public Object {
 protected:
     explicit Error(Object& prototype);
 
+    virtual void visit_edges(Visitor&) override;
+
 private:
     virtual bool is_error_object() const final { return true; }
 

Libraries/LibJS/Runtime/ExecutionContext.h

@@ -164,8 +164,8 @@ static_assert(IsTriviallyDestructible<ExecutionContext>);
     } while (0)
 
 struct StackTraceElement {
-    ExecutionContext* execution_context;
-    GC::Root<CachedSourceRange> source_range;
+    ExecutionContext* execution_context { nullptr };
+    GC::Ptr<CachedSourceRange> source_range;
 };
 
 }

Libraries/LibJS/Runtime/VM.cpp

@@ -774,11 +774,11 @@ static GC::Ptr<CachedSourceRange> get_source_range(ExecutionContext* context)
     return context->rare_data()->cached_source_range;
 }
 
-Vector<StackTraceElement> VM::stack_trace() const
+GC::ConservativeVector<StackTraceElement> VM::stack_trace() const
 {
-    Vector<StackTraceElement> stack_trace;
-    for (ssize_t i = m_execution_context_stack.size() - 1; i >= 0; i--) {
-        auto* context = m_execution_context_stack[i];
+    GC::ConservativeVector<StackTraceElement> stack_trace(heap());
+    stack_trace.ensure_capacity(m_execution_context_stack.size());
+    for (auto* context : m_execution_context_stack.in_reverse()) {
         stack_trace.append({
             .execution_context = context,
             .source_range = get_source_range(context),

Libraries/LibJS/Runtime/VM.h

@@ -58,8 +58,7 @@ class JS_API VM : public RefCounted<VM> {
     static NonnullRefPtr<VM> create();
     ~VM();
 
-    GC::Heap& heap() { return m_heap; }
-    GC::Heap const& heap() const { return m_heap; }
+    GC::Heap& heap() const { return const_cast<GC::Heap&>(m_heap); }
 
     Bytecode::Interpreter& bytecode_interpreter() { return *m_bytecode_interpreter; }
 
@@ -295,7 +294,7 @@ class JS_API VM : public RefCounted<VM> {
     Function<ThrowCompletionOr<void>(Realm&, NonnullOwnPtr<ExecutionContext>, ShadowRealm&)> host_initialize_shadow_realm;
     Function<Crypto::SignedBigInteger(Object const& global)> host_system_utc_epoch_nanoseconds;
 
-    Vector<StackTraceElement> stack_trace() const;
+    [[nodiscard]] GC::ConservativeVector<StackTraceElement> stack_trace() const;
 
 private:
     using ErrorMessages = AK::Array<Utf16String, to_underlying(ErrorMessage::__Count)>;