/usr/include/d/stdx-allocator/ is in libstdx-allocator-dev 2.77.1.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/d/stdx-allocator/stdx/
/usr/include/d/stdx-allocator/stdx/allocator/
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/affix_allocator.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/allocator_list.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/bitmapped_block.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/bucketizer.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/fallback_allocator.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/free_list.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/free_tree.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/kernighan_ritchie.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/null_allocator.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/package.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/quantizer.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/region.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/scoped_allocator.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/segregator.d
/usr/include/d/stdx-allocator/stdx/allocator/building_blocks/stats_collector.d
/usr/include/d/stdx-allocator/stdx/allocator/common.d
/usr/include/d/stdx-allocator/stdx/allocator/gc_allocator.d
/usr/include/d/stdx-allocator/stdx/allocator/internal.d
/usr/include/d/stdx-allocator/stdx/allocator/mallocator.d
/usr/include/d/stdx-allocator/stdx/allocator/mmap_allocator.d
/usr/include/d/stdx-allocator/stdx/allocator/package.d
/usr/include/d/stdx-allocator/stdx/allocator/showcase.d
/usr/include/d/stdx-allocator/stdx/allocator/typed.d