optimize string usage: use fromLatin1() less

.. to reduce allocations.

Replace fromLatin1 with QLatin1String or
with QStringBuilder where it is possible.

Change-Id: I09c7242fa7b118447b51239e2a6743a34fb3de14
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/qmltooling/qmldbg_debugger/qqmlenginedebugservice.cpp

@@ -289,10 +289,12 @@ void QQmlEngineDebugServiceImpl::buildObjectDump(QDataStream &message,
                 prop.value = expr->expression();
                 QObject *scope = expr->scopeObject();
                 if (scope) {
-                    QString methodName = QString::fromLatin1(QMetaObjectPrivate::signal(scope->metaObject(), signalHandler->signalIndex()).name());
-                    if (!methodName.isEmpty()) {
-                        prop.name = QLatin1String("on") + methodName[0].toUpper()
-                                + methodName.mid(1);
+                    const QByteArray methodName = QMetaObjectPrivate::signal(scope->metaObject(),
+                                                                             signalHandler->signalIndex()).name();
+                    const QLatin1String methodNameStr(methodName);
+                    if (methodNameStr.size() != 0) {
+                        prop.name = QLatin1String("on") + QChar(methodNameStr.at(0)).toUpper()
+                                + methodNameStr.mid(1);
                     }
                 }
             }

src/qml/jsruntime/qv4qobjectwrapper.cpp

@@ -1302,9 +1302,8 @@ static QV4::ReturnedValue CallPrecise(const QQmlObjectOrGadget &object, const QQ
     int returnType = object.methodReturnType(data, &unknownTypeError);
 
     if (returnType == QMetaType::UnknownType) {
-        QString typeName = QString::fromLatin1(unknownTypeError);
-        QString error = QStringLiteral("Unknown method return type: %1").arg(typeName);
-        return engine->throwError(error);
+        return engine->throwError(QLatin1String("Unknown method return type: ")
+                                  + QLatin1String(unknownTypeError));
     }
 
     if (data.hasArguments()) {
@@ -1319,9 +1318,8 @@ static QV4::ReturnedValue CallPrecise(const QQmlObjectOrGadget &object, const QQ
             args = object.methodParameterTypes(data.coreIndex(), &storage, &unknownTypeError);
 
         if (!args) {
-            QString typeName = QString::fromLatin1(unknownTypeError);
-            QString error = QStringLiteral("Unknown method parameter type: %1").arg(typeName);
-            return engine->throwError(error);
+            return engine->throwError(QLatin1String("Unknown method parameter type: ")
+                                      + QLatin1String(unknownTypeError));
         }
 
         if (args[0] > callArgs->argc) {

src/qml/jsruntime/qv4variantobject.cpp

@@ -138,8 +138,11 @@ QV4::ReturnedValue VariantPrototype::method_toString(CallContext *ctx)
     if (!o)
         return Encode::undefined();
     QString result = o->d()->data.toString();
-    if (result.isEmpty() && !o->d()->data.canConvert(QVariant::String))
-        result = QStringLiteral("QVariant(%0)").arg(QString::fromLatin1(o->d()->data.typeName()));
+    if (result.isEmpty() && !o->d()->data.canConvert(QVariant::String)) {
+        result = QLatin1String("QVariant(")
+                 + QLatin1String(o->d()->data.typeName())
+                 + QLatin1Char(')');
+    }
     return Encode(ctx->d()->engine->newString(result));
 }
 

src/qmltest/quicktestresult.cpp

@@ -519,7 +519,7 @@ void QuickTestResult::stringify(QQmlV4Function *args)
     if (result.isEmpty()) {
         QString tmp = value->toQStringNoThrow();
         if (value->as<QV4::ArrayObject>())
-            result.append(QString::fromLatin1("[%1]").arg(tmp));
+            result += QLatin1Char('[') + tmp + QLatin1Char(']');
         else
             result.append(tmp);
     }