QBuffer: optimize setData(ptr, n)

The old code always created a new QByteArray, always allocating
memory.

The new call assigns the data to the existing QByteArray, enabling
potential re-use of the internal QByteArray's buffer. Since QByteArray
is missing the STL-style assign() function, abuse replace() for this
task.

Change-Id: I357f11bad0a976d4d0fb2faeb93f8b2262fa5a65
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/io/qbuffer.cpp

@@ -267,8 +267,6 @@ void QBuffer::setData(const QByteArray &data)
 }
 
 /*!
-    \fn void QBuffer::setData(const char *data, qsizetype size)
-
     \overload
 
     Sets the contents of the internal buffer to be the first \a size
@@ -277,6 +275,16 @@ void QBuffer::setData(const QByteArray &data)
     \note In Qt versions prior to 6.5, this function took the length as
     an \c{int} parameter, potentially truncating sizes.
 */
+void QBuffer::setData(const char *data, qsizetype size)
+{
+    Q_D(QBuffer);
+    if (isOpen()) {
+        qWarning("QBuffer::setData: Buffer is open");
+        return;
+    }
+    d->buf->replace(qsizetype(0), d->buf->size(), // ### QByteArray lacks assign(ptr, n)
+                    data, size);
+}
 
 /*!
    \reimp

src/corelib/io/qbuffer.h

@@ -36,8 +36,7 @@ public:
 #if QT_CORE_REMOVED_SINCE(6, 5) && QT_POINTER_SIZE != 4
     void setData(const char *data, int len) { setData(data, qsizetype(len)); }
 #endif
-    void setData(const char *data, qsizetype len)
-    { setData(QByteArray(data, len)); }
+    void setData(const char *data, qsizetype len);
     const QByteArray &data() const;
 
     bool open(OpenMode openMode) override;