QVarLengthArray: add rvalue overload of append/push_back

Improves performance when appending temporaries, esp. since
the aliasing fix in the lvalue overload in 0f730ef made that
overload correct, but a bit slower across reallocs.

The unit tests already also pass rvalues, so the function is
covered in the existing tests.

[ChangeLog][QtCore][QVarLengthArray] Added rvalue overloads
of append() and push_back().

Change-Id: If3a6970f03a160cba5b42d33d32d3d18948f6ce3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -165,6 +165,16 @@ public:
         }
     }
 
+    void append(T &&t) {
+        if (s == a)
+            realloc(s, s << 1);
+        const int idx = s++;
+        if (QTypeInfo<T>::isComplex)
+            new (ptr + idx) T(std::move(t));
+        else
+            ptr[idx] = std::move(t);
+    }
+
     void append(const T *buf, int size);
     inline QVarLengthArray<T, Prealloc> &operator<<(const T &t)
     { append(t); return *this; }
@@ -218,6 +228,7 @@ public:
     // STL compatibility:
     inline bool empty() const { return isEmpty(); }
     inline void push_back(const T &t) { append(t); }
+    void push_back(T &&t) { append(std::move(t)); }
     inline void pop_back() { removeLast(); }
     inline T &front() { return first(); }
     inline const T &front() const { return first(); }

src/corelib/tools/qvarlengtharray.qdoc

@@ -302,6 +302,34 @@
     Provided for STL-compatibility.
 */
 
+/*!
+    \fn void QVarLengthArray::append(T &&t)
+    \overload append
+    \since 5.9
+
+    \note Unlike the lvalue overload of append(), passing a reference to
+    an object that is already an element of \c *this leads to undefined
+    behavior:
+
+    \code
+    vla.append(std::move(vla[0])); // BUG: passing an object that is already in the container
+    \endcode
+*/
+
+/*!
+    \fn void QVarLengthArray::push_back(T &&t)
+    \overload push_back
+    \since 5.9
+
+    \note Unlike the lvalue overload of push_back(), passing a reference to
+    an object that is already an element of \c *this leads to undefined
+    behavior:
+
+    \code
+    vla.push_back(std::move(vla[0])); // BUG: passing an object that is already in the container
+    \endcode
+*/
+
 /*!
     \fn inline void QVarLengthArray::removeLast()
     \since 4.5