Utils: Avoid copy on FileName::toString()

Takes ~6% of project loading

Change-Id: Id277f6cc9d5666b9a383419959cfd37b936437f8
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/libs/utils/fileutils.cpp

@@ -548,9 +548,9 @@ QFileInfo FileName::toFileInfo() const
 }
 
 /// \returns a QString for passing on to QString based APIs
-QString FileName::toString() const
+const QString &FileName::toString() const
 {
-    return QString(*this);
+    return *this;
 }
 
 /// \returns a QString to display to the user

src/libs/utils/fileutils.h

@@ -71,7 +71,7 @@ public:
     static FileName fromLatin1(const QByteArray &filename);
     static FileName fromUserInput(const QString &filename);
     static FileName fromUtf8(const char *filename, int filenameSize = -1);
-    QString toString() const;
+    const QString &toString() const;
     QString toUserOutput() const;
     QString fileName(int pathComponents = 0) const;
     bool exists() const;