fn build_region_mapping(
source_regions: &[RegionDescriptor],
target_regions: &[RegionDescriptor],
transition_map: &[Vec<usize>],
) -> HashMap<RegionId, Vec<RegionId>>fn build_region_mapping(
source_regions: &[RegionDescriptor],
target_regions: &[RegionDescriptor],
transition_map: &[Vec<usize>],
) -> HashMap<RegionId, Vec<RegionId>>