LibJS: Cache the formatted Error.prototype.stack string

Generating the stack trace string is expensive as it involves
formatting each frame with function names, file paths, and line
numbers. Since the stack trace is immutable after error creation,
we can cache the formatted string on first access.

This matches the caching behavior of V8 and JavaScriptCore, which
also return the same cached string on subsequent accesses to the
stack property.

Libraries/LibJS/Runtime/Error.cpp

@@ -61,6 +61,7 @@ Error::Error(Object& prototype)
 void Error::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);
+    visitor.visit(m_cached_string);
     for (auto& frame : m_traceback)
         visitor.visit(frame.cached_source_range);
 }

Libraries/LibJS/Runtime/Error.h

@@ -48,6 +48,9 @@ public:
 
     Vector<TracebackFrame, 32> const& traceback() const { return m_traceback; }
 
+    void set_cached_string(GC::Ref<PrimitiveString> string) { m_cached_string = string; }
+    GC::Ptr<PrimitiveString> cached_string() const { return m_cached_string; }
+
 protected:
     explicit Error(Object& prototype);
 
@@ -58,6 +61,8 @@ private:
 
     void populate_stack();
     Vector<TracebackFrame, 32> m_traceback;
+
+    GC::Ptr<PrimitiveString> m_cached_string;
 };
 
 template<>

Libraries/LibJS/Runtime/ErrorPrototype.cpp

@@ -83,6 +83,12 @@ JS_DEFINE_NATIVE_FUNCTION(ErrorPrototype::stack_getter)
 
     auto& error = static_cast<Error&>(*this_object);
 
+    // OPTIMIZATION: Avoid recomputing the stack string if we already have it cached.
+    //               At least one major engine does this as well, so it's not expected that changing
+    //               the name or message properties updates the stack string.
+    if (error.cached_string())
+        return error.cached_string();
+
     // 4. Return ? GetStackString(error).
     // NOTE: These steps are not implemented based on the proposal, but to roughly follow behavior of other browsers.
 
@@ -100,7 +106,9 @@ JS_DEFINE_NATIVE_FUNCTION(ErrorPrototype::stack_getter)
         ? move(name)
         : MUST(String::formatted("{}: {}", name, message));
 
-    return PrimitiveString::create(vm, MUST(String::formatted("{}\n{}", header, error.stack_string())));
+    auto string = PrimitiveString::create(vm, MUST(String::formatted("{}\n{}", header, error.stack_string())));
+    error.set_cached_string(string);
+    return string;
 }
 
 // B.1.2 set Error.prototype.stack ( value ), https://tc39.es/proposal-error-stacks/#sec-set-error.prototype-stack