static File renameFile_assertTrue(File a, File b) ctex {
  if (a.equals(b)) ret b; // no rename necessary
  if (!a.exists()) fail("Source file not found: " + f2s(a));
  if (b.exists()) fail("Target file exists: " + f2s(b));
  mkdirsForFile(b);
  ifdef renameFile_useNIO
    java.nio.file.Files.move(a.toPath(), b.toPath());
  endifdef
  ifndef renameFile_useNIO
  if (!a.renameTo(b))
    fail("Can't rename " + f2s(a) + " to " + f2s(b));
  endifndef
  ret b;
}

Began life as a copy of #1002706