This rather specialized function walks each region r that appear in ty and invokes relate_op(r_encl, r) for each one. r_encl here is the region of any enclosing &'r T pointer. If there is no enclosing pointer, and opt_region is Some, then opt_region.get() is used instead. Otherwise, no callback occurs at all).

Here are some examples to give you an intution:

• relate_nested_regions(Some('r1), &'r2 uint) invokes
  • relate_op('r1, 'r2)
• relate_nested_regions(Some('r1), &'r2 &'r3 uint) invokes
  • relate_op('r1, 'r2)
  • relate_op('r2, 'r3)
• relate_nested_regions(None, &'r2 &'r3 uint) invokes
  • relate_op('r2, 'r3)
• relate_nested_regions(None, &'r2 &'r3 &'r4 uint) invokes
  • relate_op('r2, 'r3)
  • relate_op('r2, 'r4)
  • relate_op('r3, 'r4)

This function is used in various pieces of code because we enforce the constraint that a region pointer cannot outlive the things it points at. Hence, in the second example above, 'r2 must be a subregion of 'r3.