3 files changed, 43 insertions, 21 deletions

diff --git a/src/corelib/text/qbytearray.h b/src/corelib/text/qbytearray.h
index 8989eb27a6..dbbbd2f178 100644
--- a/src/corelib/text/qbytearray.h
+++ b/src/corelib/text/qbytearray.h
@@ -473,10 +473,10 @@ public:
     bool isNull() const noexcept;
 
     inline DataPointer &data_ptr() { return d; }
-    explicit inline QByteArray(const DataPointer &dd)
-        : d(dd)
-    {
-    }
+#if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)
+    explicit inline QByteArray(const DataPointer &dd) : d(dd) {}
+#endif
+    explicit inline QByteArray(DataPointer &&dd) : d(std::move(dd)) {}
 
 private:
     void reallocData(qsizetype alloc, QArrayData::AllocationOption option);
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 23927b0e84..57bed1240f 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -5269,6 +5269,7 @@ QByteArray QtPrivate::convertToLatin1(QStringView string)
     return qt_convert_to_latin1(string);
 }
 
+Q_NEVER_INLINE
 static QByteArray qt_convert_to_latin1(QStringView string)
 {
     if (Q_UNLIKELY(string.isNull()))
@@ -5290,27 +5291,26 @@ QByteArray QString::toLatin1_helper_inplace(QString &s)
 
     // We can return our own buffer to the caller.
     // Conversion to Latin-1 always shrinks the buffer by half.
-    const char16_t *data = s.d.data();
-    qsizetype length = s.d.size;
-
-    // Move the d pointer over to the bytearray.
-    // Kids, avert your eyes. Don't try this at home.
+    // This relies on the fact that we use QArrayData for everything behind the scenes
 
-    // this relies on the fact that we use QArrayData for everything behind the scenes which has the same layout
-    static_assert(sizeof(QByteArray::DataPointer) == sizeof(QString::DataPointer), "sizes have to be equal");
-    QByteArray::DataPointer ba_d(reinterpret_cast<QByteArray::Data *>(s.d.d_ptr()), reinterpret_cast<char *>(s.d.data()), length);
-    ba_d.ref();
-    s.clear();
+    // First, do the in-place conversion. Since isDetached() == true, the data
+    // was allocated by QArrayData, so the null terminator must be there.
+    qsizetype length = s.size();
+    char16_t *sdata = s.d->data();
+    Q_ASSERT(sdata[length] == u'\0');
+    qt_to_latin1(reinterpret_cast<uchar *>(sdata), sdata, length + 1);
 
-    char *ddata = ba_d.data();
+    // Move the internals over to the byte array.
+    // Kids, avert your eyes. Don't try this at home.
+    auto ba_d = std::move(s.d).reinterpreted<char>();
 
-    // multiply the allocated capacity by sizeof(char16_t)
-    ba_d.d_ptr()->alloc *= sizeof(char16_t);
+    // Some sanity checks
+    Q_ASSERT(ba_d.d->allocatedCapacity() >= ba_d.size);
+    Q_ASSERT(s.isNull());
+    Q_ASSERT(s.isEmpty());
+    Q_ASSERT(s.constData() == QString().constData());
 
-    // do the in-place conversion
-    qt_to_latin1(reinterpret_cast<uchar *>(ddata), data, length);
-    ddata[length] = '\0';
-    return QByteArray(ba_d);
+    return QByteArray(std::move(ba_d));
 }
 
 /*!
diff --git a/src/corelib/tools/qarraydatapointer.h b/src/corelib/tools/qarraydatapointer.h
index 4d83b890cc..dda3cf13eb 100644
--- a/src/corelib/tools/qarraydatapointer.h
+++ b/src/corelib/tools/qarraydatapointer.h
@@ -168,6 +168,28 @@ public:
 
     /*! \internal
 
+        Reinterprets the data of this QArrayDataPointer to type X. It's the
+        caller's responsibility to ensure that the data contents are valid and
+        properly aligned, particularly if T and X are not trivial types (i.e,
+        don't do that). The current size is kept and the allocated capacity is
+        updated to account for the difference in the element type's size.
+
+        This is used in QString::fromLatin1 to perform in-place conversion of
+        QString to QByteArray.
+    */
+    template <typename X> QArrayDataPointer<X> reinterpreted() &&
+    {
+        if (sizeof(T) != sizeof(X)) {
+            Q_ASSERT(!d->isShared());
+            d->alloc = d->alloc * sizeof(T) / sizeof(X);
+        }
+        auto od = reinterpret_cast<QTypedArrayData<X> *>(std::exchange(d, nullptr));
+        auto optr = reinterpret_cast<X *>(std::exchange(ptr, nullptr));
+        return { od, optr, std::exchange(size, 0) };
+    }
+
+    /*! \internal
+
         Detaches this (optionally) and grows to accommodate the free space for
         \a n elements at the required side. The side is determined from \a pos.