String Views: optimize the QByteArray/QString constructors

By using {QString,QByteArray}::begin() instead of data() (for const
objectst); both begin() methods, respectively [1], return nullptr for a
const null {QString,QByteArray}. Whereas data() returns &_empty if
QT5_NULL_STRINGS is defined.

[1] QByteArray::begin() since a116b2ddfc
QString::begin() since 287ace562e

[ChangeLog][QtCore][QLatin1StringView] Creation of QLatin1StringView
from QByteArray or QByteArrayView now preserves the origin's isNull()
status. This matches the behavior of creating a QUtf8StringView from
those types.

Change-Id: Icbdfe282e315c08ca6a17201bcd32c1a3bba8bf5
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 221d8fdfb14c16472267320f859105f1a3834952)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qbytearrayview.h

@@ -166,7 +166,7 @@ public:
 #else
     template <typename ByteArray, if_compatible_qbytearray_like<ByteArray> = true>
     QByteArrayView(const ByteArray &ba) noexcept
-        : QByteArrayView(ba.isNull() ? nullptr : ba.data(), qsizetype(ba.size())) {}
+        : QByteArrayView{ba.begin(), ba.size()} {}
 #endif
 
     template <typename Container, if_compatible_container<Container> = true>

src/corelib/text/qlatin1stringview.h

@@ -40,7 +40,7 @@ public:
         : QLatin1StringView(f, qsizetype(l - f)) {}
     constexpr QLatin1StringView(const char *s, qsizetype sz) noexcept : m_data(s), m_size(sz) {}
     explicit QLatin1StringView(const QByteArray &s) noexcept
-        : QLatin1StringView(s.constData(), s.size()) {}
+        : QLatin1StringView{s.begin(), s.size()} {}
     constexpr explicit QLatin1StringView(QByteArrayView s) noexcept
         : QLatin1StringView(s.constData(), s.size()) {}
 #else
@@ -52,7 +52,7 @@ public:
     constexpr QLatin1String(const char *f, const char *l)
         : QLatin1String(f, qsizetype(l - f)) {}
     constexpr QLatin1String(const char *s, qsizetype sz) noexcept : m_size(sz), m_data(s) {}
-    explicit QLatin1String(const QByteArray &s) noexcept : m_size(s.size()), m_data(s.constData()) {}
+    explicit QLatin1String(const QByteArray &s) noexcept : QLatin1String(s.begin(), s.size()) {}
     constexpr explicit QLatin1String(QByteArrayView s) noexcept : m_size(s.size()), m_data(s.data()) {}
 #endif // !Q_L1S_VIEW_IS_PRIMARY
 

src/corelib/text/qstring.h

@@ -1213,9 +1213,9 @@ template <bool UseChar8T>
 //
 
 QAnyStringView::QAnyStringView(const QByteArray &str) noexcept
-    : QAnyStringView{str.isNull() ? nullptr : str.data(), str.size()} {}
+    : QAnyStringView{str.begin(), str.size()} {}
 QAnyStringView::QAnyStringView(const QString &str) noexcept
-    : QAnyStringView{str.isNull() ? nullptr : str.data(), str.size()} {}
+    : QAnyStringView{str.begin(), str.size()} {}
 
 QString QAnyStringView::toString() const
 { return QtPrivate::convertToQString(*this); }

src/corelib/text/qstringview.h

@@ -160,7 +160,7 @@ public:
 #else
     template <typename String, if_compatible_qstring_like<String> = true>
     QStringView(const String &str) noexcept
-        : QStringView(str.isNull() ? nullptr : str.data(), qsizetype(str.size())) {}
+        : QStringView{str.begin(), str.size()} {}
 #endif
 
     template <typename Container, if_compatible_container<Container> = true>

src/corelib/text/qutf8stringview.h

@@ -181,7 +181,7 @@ public:
 #else
     template <typename String, if_compatible_qstring_like<String> = true>
     QBasicUtf8StringView(const String &str) noexcept
-        : QBasicUtf8StringView(str.isNull() ? nullptr : str.data(), qsizetype(str.size())) {}
+        : QBasicUtf8StringView{str.begin(), str.size()} {}
 #endif
 
     template <typename Container, if_compatible_container<Container> = true>

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

@@ -272,12 +272,10 @@ void tst_QLatin1StringView::nullString()
         QVERIFY(null.isNull());
 
         QLatin1StringView l1(null);
-        QEXPECT_FAIL("", "null QByteArrays become non-null QLatin1Strings...", Continue);
         QCOMPARE(static_cast<const void*>(l1.data()), static_cast<const void*>(nullptr));
         QCOMPARE(l1.size(), 0);
 
         QString s = l1;
-        QEXPECT_FAIL("", "null QByteArrays become non-null QLatin1Strings become non-null QStrings...", Continue);
         QVERIFY(s.isNull());
     }
 }