QLatin1StringView: add toUtf8()

A more efficient way of transencoding from Latin-1 to UTF-8 without
going through UTF-16 (QString).

[ChangeLog][QtCore][QLatin1StringView] Added toUtf8(), which can convert
without going through QString first.

Change-Id: I937f9536a473b990ba05fffd3670234055d0e46c
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/text/qlatin1stringview.h

@@ -58,6 +58,7 @@ public:
 #endif // !Q_L1S_VIEW_IS_PRIMARY
 
     inline QString toString() const;
+    QByteArray toUtf8() const { return QtPrivate::convertToUtf8(*this); }
 
     constexpr const char *latin1() const noexcept { return m_data; }
     constexpr qsizetype size() const noexcept { return m_size; }

src/corelib/text/qlatin1stringview.qdoc

@@ -270,6 +270,18 @@
     \code
     return QString(*this);
     \endcode
+
+    \sa toUtf8()
+*/
+
+/*!
+    \fn QByteArray QLatin1StringView::toUtf8() const
+    \since 6.9
+
+    Returns a UTF-8 representation of the string as a QByteArray. This function
+    is more efficient than converting first to QString.
+
+    \sa toString(), QString::toUtf8()
 */
 
 /*! \fn const char *QLatin1StringView::latin1() const

src/corelib/text/qstring.cpp

@@ -5712,6 +5712,7 @@ QByteArray QtPrivate::convertToUtf8(QLatin1StringView string)
     if (Q_UNLIKELY(string.isNull()))
         return QByteArray();
 
+    // create a QByteArray with the worst case scenario size
     QByteArray ba(string.size() * 2, Qt::Uninitialized);
     const qsizetype sz = QUtf8::convertFromLatin1(ba.data(), string) - ba.data();
     ba.truncate(sz);

tests/auto/corelib/text/qlatin1stringview/tst_qlatin1stringview.cpp

@@ -33,6 +33,8 @@ private Q_SLOTS:
     void count();
     void indexOf_data();
     void indexOf();
+    void toUtf8_data();
+    void toUtf8();
 };
 
 void tst_QLatin1StringView::constExpr()
@@ -512,6 +514,43 @@ void tst_QLatin1StringView::indexOf()
     QCOMPARE(haystack.indexOf(needle, from, Qt::CaseInsensitive), (qsizetype)indexCaseInsensitive);
 }
 
+void tst_QLatin1StringView::toUtf8_data()
+{
+    QTest::addColumn<QByteArray>("input");
+    QTest::newRow("null") << QByteArray();
+    QTest::newRow("empty") << QByteArray("");
+
+    for (int i = 0; i < 256; ++i) {
+        char c = i;
+        QTest::addRow("char-0x%02x", i) << QByteArray(1, c);
+    }
+
+    QByteArray ba = "abcd";
+    for (int i = 0; i < 6; ++i) {
+        QTest::addRow("ascii-%d", int(ba.size())) << ba;
+        ba += ba;
+        QTest::addRow("ascii-%d", int(ba.size()) - 1) << ba.left(ba.size() - 1);
+    }
+
+    ba = "\xe0""abcdef\xff";
+    for (int i = 0; i < 6; ++i) {
+        QTest::addRow("nonascii-%d", int(ba.size())) << ba;
+        ba += ba;
+        QTest::addRow("nonascii-%d", int(ba.size()) - 1) << ba.left(ba.size() - 1);
+    }
+}
+
+void tst_QLatin1StringView::toUtf8()
+{
+    QFETCH(QByteArray, input);
+    QLatin1StringView sv(input);
+    QByteArray expected = sv.toString().toUtf8();
+
+    QByteArray result = sv.toUtf8();
+    QCOMPARE(result.isNull(), sv.isNull());
+    QCOMPARE(result, expected);
+}
+
 QTEST_APPLESS_MAIN(tst_QLatin1StringView)
 
 #include "tst_qlatin1stringview.moc"