The field order of structs is unspecified by default, to allow, e.g. reordering to the smallest size, or even randomising for security/fuzz testing. One can op

The field order of structs is unspecified by default, to allow, e.g. reordering to the smallest size, or even randomising for security/fuzz testing. One can opt-in to a fixed in-order layout with `#[repr(C)]` and even avoid padding for such a struct with `#[repr(C, packed)]`. NB. in general it is impossible to pack a struct perfectly manually, e.g. generic structs can have their order for minimal size change depending on what types they're used with. It can even be somewhat platform specific, with e.g. pointers changing size/alignment on 32-bit vs. 64-bit computers (although pointers probably aren't problematic, since they're switching between two adjacent classes).