PVS-Studio warning V587 for Firefox

void nsRegion::SimplifyOutwardByArea(uint32_t aThreshold)
{
  ....
  topRects = destRect;
  bottomRects = bottomRectsEnd;
  destRect = topRects;
  ....
}

This suspicious code was found in Firefox project by PVS-Studio static code analyzer.
Warning message is:
V587 An odd sequence of assignments of this kind: A = B; B = A;. Check lines: 358, 360. nsregion.cpp 360

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/