This function populates the region map's free_region_map. It walks over the transformed self type and argument types for each function just before we check the body of that function, looking for types where you have a borrowed pointer to other borrowed data (e.g., &'a &'b [uint]. We do not allow references to outlive the things they point at, so we can assume that 'a <= 'b.

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).