QVarLengthArray: Extract Method QtPrivate::q_rotate()

It seems like we'll need this in lots of other places, too.

Pick-to: 6.5 6.4 6.2 5.15
Change-Id: I767495c2eb02a2fc85b6f835ad9003fa89315c7f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qcontainertools_impl.h

@@ -86,6 +86,26 @@ void q_uninitialized_relocate_n(T* first, N n, T* out)
 
 QT_WARNING_POP
 
+/*!
+    \internal
+
+    A wrapper around std::rotate(), with an optimization for
+    Q_RELOCATABLE_TYPEs. We omit the return value, as it would be more work to
+    compute in the Q_RELOCATABLE_TYPE case and, unlike std::rotate on
+    ForwardIterators, callers can compute the result in constant time
+    themselves.
+*/
+template <typename T>
+void q_rotate(T *first, T *mid, T *last)
+{
+    if constexpr (QTypeInfo<T>::isRelocatable) {
+        const auto cast = [](T *p) { return reinterpret_cast<uchar*>(p); };
+        std::rotate(cast(first), cast(mid), cast(last));
+    } else {
+        std::rotate(first, mid, last);
+    }
+}
+
 /*!
     \internal
     Copies all elements, except the ones for which \a pred returns \c true, from

src/corelib/tools/qvarlengtharray.h

@@ -916,12 +916,7 @@ Q_OUTOFLINE_TEMPLATE auto QVLABase<T>::emplace_impl(qsizetype prealloc, void *ar
     emplace_back_impl(prealloc, array, std::forward<Args>(args)...);
     const auto b = begin() + offset;
     const auto e = end();
-    if constexpr (QTypeInfo<T>::isRelocatable) {
+    QtPrivate::q_rotate(b, e - 1, e);
-        auto cast = [](T *p) { return reinterpret_cast<uchar*>(p); };
-        std::rotate(cast(b), cast(e - 1), cast(e));
-    } else {
-        std::rotate(b, e - 1, e);
-    }
     return b;
 }