QString: fix lifetime issues with QRegularExpression APIs

QString has several functions taking a QRegularExpression: indexOf(),
contains(), and so on. Some of those have an out-argument of type
QRegularExpressionMatch, to report the details of the match (if any).
For instance:

  QRegularExpression re(...);
  QRegularExpressionMatch match;

  if (string.contains(re, &match))
    use(match);

The code used to route the implementation of these functions through
QStringView (which has the very same functions). This however opens
up a lifetime problem with temporary strings:

  if (getString().contains(re, &match))
    use(match); // match is dangling

Here `match` is dangling because it is referencing data into the
destroyed temporary -- nothing is keeping the string alive. This is
against the rules we've decided for Qt, and it's also asymmetric with
the corresponding code that uses QRegularExpression directly instead:

  match = re.match(getString());
  if (match.hasMatch())
    use(match); // not dangling

... although we've documented not to do this. (In light of the decision
we've made w.r.t. temporaries, the documentation is wrong anyways.)
Here QRE takes a copy of the string and stores it in the match object,
thus keeping it alive.

Hence, extend the implementation of the QString functions to keep a
(shallow) copy of the string. To keep the code shared as much as
possible with QStringView, in theory one could have a function taking a
std::variant<QString, QStringView> and that uses the currently active
member. However I've found that std::variant here creates some abysmal
codegen, so instead I went for a simpler approach -- pass a QStringView
and an optional pointer to a QString. Use the latter if it's loaded.

QStringView has some inline code that calls into exported functions, so
I can't change the signature of them without breaking BC; I'm instead
adding new unexported functions and a Qt 7 note to unify them.

Change-Id: I7c65885a84069d0fbb902dcc96ddff543ca84562
Fixes: QTBUG-103940
Pick-to: 6.2 6.3 6.4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 40 insertions, 0 deletions

diff --git a/tests/auto/corelib/text/qstring/tst_qstring.cpp b/tests/auto/corelib/text/qstring/tst_qstring.cpp
index 467d5b0b39..239b3498a9 100644
--- a/tests/auto/corelib/text/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/text/qstring/tst_qstring.cpp
@@ -537,6 +537,7 @@ private slots:
 #if QT_CONFIG(regularexpression)
     void split_regularexpression_data();
     void split_regularexpression();
+    void regularexpression_lifetime();
 #endif
     void fromUtf16_data();
     void fromUtf16();
@@ -6244,6 +6245,45 @@ void tst_QString::split_regularexpression()
     split_regexp<QStringList, QRegularExpression>(string, pattern, result);
     split_regexp<QList<QStringView>, QRegularExpression>(string, pattern, result);
 }
+
+// Test that rvalue strings (e.g. temporaries) are kept alive in
+// QRegularExpression-related APIs
+void tst_QString::regularexpression_lifetime()
+{
+    const auto getString = [] {
+        // deliberately heap-allocated
+        return QString(QLatin1String("the quick brown fox jumps over the lazy dog"));
+    };
+
+    QRegularExpression re("\\w{5}");
+
+    {
+        QString s = getString();
+        QRegularExpressionMatch match;
+        const bool contains = std::move(s).contains(re, &match);
+        s.fill('X'); // NOLINT(bugprone-use-after-move)
+        QVERIFY(contains);
+        QCOMPARE(match.capturedView(), u"quick");
+    }
+
+    {
+        QString s = getString();
+        QRegularExpressionMatch match;
+        const auto index = std::move(s).indexOf(re, 0, &match);
+        s.fill('X'); // NOLINT(bugprone-use-after-move)
+        QCOMPARE(index, 4);
+        QCOMPARE(match.capturedView(), u"quick");
+    }
+
+    {
+        QString s = getString();
+        QRegularExpressionMatch match;
+        const auto lastIndex = std::move(s).lastIndexOf(re, &match);
+        s.fill('X'); // NOLINT(bugprone-use-after-move)
+        QCOMPARE(lastIndex, 20);
+        QCOMPARE(match.capturedView(), u"jumps");
+    }
+}
 #endif
 
 void tst_QString::fromUtf16_data()