QStringView: add internal qToStringViewIgnoringNull()

As long as a null QString still returns non-null data(), QStringView's
QString constructor needs to call isNull(). That's a branch we often
can do without, so add an internal function that bypasses this
correctness check.

It's internal, since we might have a Q6String that returns nullptr
data() when null, which will remove the need for this function.

What the QStringView(QString) ctor does will also have to be
re-evaluated come Qt 6, but for now, keep the public QString-
QStringView conversion correct.

Change-Id: I35dc7383bc3bd018f46aeec429185135a38ddcef
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qstringview.cpp

@@ -704,4 +704,19 @@ QT_BEGIN_NAMESPACE
     \sa toUtf8(), toLatin1(), toLocal8Bit(), QTextCodec
 */
 
+/*!
+    \fn qToStringViewIgnoringNull(const QStringLike &s);
+    \since 5.10
+    \internal
+
+    Convert \a s to a QStringView ignoring \c{s.isNull()}.
+
+    Returns a string-view that references \a{s}' data, but is never null.
+
+    This is a faster way to convert a QString or QStringRef to a QStringView,
+    if null QStrings can legitimately be treated as empty ones.
+
+    \sa QString::isNull(), QStringRef::isNull(), QStringView
+*/
+
 QT_END_NAMESPACE

src/corelib/tools/qstringview.h

@@ -272,6 +272,12 @@ private:
 };
 Q_DECLARE_TYPEINFO(QStringView, Q_MOVABLE_TYPE);
 
+template <typename QStringLike, typename std::enable_if<
+    std::is_same<QStringLike, QString>::value || std::is_same<QStringLike, QStringRef>::value,
+    bool>::type = true>
+inline QStringView qToStringViewIgnoringNull(const QStringLike &s) Q_DECL_NOTHROW
+{ return QStringView(s.data(), s.size()); }
+
 QT_END_NAMESPACE
 
 #endif /* QSTRINGVIEW_H */