Add QStringView::toWCharArray() to match QString

QCollator needs it to add support for QStringView.
In any case, it extends the mirror of QString's API.
Naturally, we can reimplement QString's version using it.

Change-Id: I5a23a3f2a98c7d59597b5e935542a93764b5e350
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/tools/qstring.cpp

@@ -2086,7 +2086,7 @@ int QString::toUcs4_helper(const ushort *uc, int length, uint *out)
 
   \note This function does not append a null character to the array.
 
-  \sa utf16(), toUcs4(), toLatin1(), toUtf8(), toLocal8Bit(), toStdWString()
+  \sa utf16(), toUcs4(), toLatin1(), toUtf8(), toLocal8Bit(), toStdWString(), QStringView::toWCharArray()
 */
 
 /*! \fn QString::QString(const QString &other)

src/corelib/tools/qstring.h

@@ -1021,12 +1021,7 @@ QT_WARNING_DISABLE_INTEL(111)   // "statement is unreachable"
 
 inline int QString::toWCharArray(wchar_t *array) const
 {
-    if (sizeof(wchar_t) == sizeof(QChar)) {
-        memcpy(array, d->data(), sizeof(QChar) * size());
-        return size();
-    } else {
-        return toUcs4_helper(d->data(), size(), reinterpret_cast<uint *>(array));
-    }
+    return QStringView(*this).toWCharArray(array);
 }
 
 QT_WARNING_POP

src/corelib/tools/qstringview.cpp

@@ -38,6 +38,7 @@
 ****************************************************************************/
 
 #include "qstringview.h"
+#include "qstring.h"
 
 QT_BEGIN_NAMESPACE
 
@@ -794,4 +795,34 @@ QT_BEGIN_NAMESPACE
     \sa QString::isRightToLeft()
 */
 
+/*!
+    \since 5.14
+
+    Transcribes this string into the given \a array.
+
+    Caller is responsible for ensuring \a array is large enough to hold the \t
+    wchar_t encoding of this string (allocating the array with the same length
+    as the string is always sufficient). The array is encoded in UTF-16 on
+    platforms where \t wchar_t is 2 bytes wide (e.g. Windows); otherwise (Unix
+    systems), \t wchar_t is assumed to be 4 bytes wide and the data is written
+    in UCS-4.
+
+    \note This function writes no null terminator to the end of \a array.
+
+    Returns the number of \t wchar_t entries written to \a array.
+
+    \sa QString::toWCharArray()
+*/
+
+int QStringView::toWCharArray(wchar_t *array) const
+{
+    if (sizeof(wchar_t) == sizeof(QChar)) {
+        memcpy(array, data(), sizeof(QChar) * size());
+        return size();
+    } else {
+        return QString::toUcs4_helper(reinterpret_cast<const ushort *>(data()), int(size()),
+                                      reinterpret_cast<uint *>(array));
+    }
+}
+
 QT_END_NAMESPACE

src/corelib/tools/qstringview.h

@@ -272,6 +272,8 @@ public:
     Q_REQUIRED_RESULT bool isRightToLeft() const Q_DECL_NOTHROW
     { return QtPrivate::isRightToLeft(*this); }
 
+    Q_REQUIRED_RESULT Q_CORE_EXPORT int toWCharArray(wchar_t *array) const;
+
     //
     // STL compatibility API:
     //