QtGlobal: add qssize_t

It replaces Posix' ssize_t in cross-platform code. Most importantly,
it allows QStringView users to refer to QStringView::size_type by
a shorter, less intimidating name.

Change-Id: I2128fefd2ffdd258b0b55cd90802a23c6bc033f6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -773,6 +773,21 @@ Q_STATIC_ASSERT_X(QT_POINTER_SIZE == sizeof(void *), "QT_POINTER_SIZE defined in
     \sa quintptr, qint32, qint64
 */
 
+/*!
+    \typedef qssize_t
+    \relates <QtGlobal>
+    \since 5.10
+
+    Integral type providing Posix' \c ssize_t for all platforms.
+
+    This type is guaranteed to be the same size as a \c size_t on all
+    platforms supported by Qt.
+
+    Note that qssize_t is signed. Use \c size_t for unsigned values.
+
+    \sa qptrdiff
+*/
+
 /*!
     \enum QtMsgType
     \relates <QtGlobal>

src/corelib/global/qglobal.h

@@ -451,6 +451,7 @@ typedef QIntegerForSize<Q_PROCESSOR_WORDSIZE>::Unsigned qregisteruint;
 typedef QIntegerForSizeof<void*>::Unsigned quintptr;
 typedef QIntegerForSizeof<void*>::Signed qptrdiff;
 typedef qptrdiff qintptr;
+using qssize_t = QIntegerForSizeof<std::size_t>::Signed;
 
 /* moc compats (signals/slots) */
 #ifndef QT_MOC_COMPAT