in rtl/inc/karray.h [301:315]
PAGED bool insertAt(size_t index, T &&t)
{
if (index > m_numElements)
return false;
if (!grow(m_numElements+1))
return false;
if (index < m_numElements)
moveElements((ULONG)index, (ULONG)(index+1), (ULONG)(m_numElements - index));
new(wistd::addressof(_p[index])) T(wistd::move(t));
++m_numElements;
return true;
}