fieldparentptr.

The zig allocators used to use this because it enabled allocator interfaces without type erasure, but it was found to have a minor but real performance penalty as it is impossible for any compiler to optimize for this in scenarios that are useful for allocators.

Other interfaces might actually have the opposite performance preference

If you control every implementation (ie you aren’t writing a library where others will implement your interfaces), then tagged unions are a simple way to accomplish this. See the bottom of this page: https://www.openmymind.net/Zig-Interfaces/