C structs are generally assumed to be laid out in the order defined with proper padding automatically inserted (as most platforms at best dislike unaligned accesses). Alignment information should only be necessary if you want to pack the structure or need to apply wider than standard alignment (e.g. need to align a 32 or 64b type to 128 bytes).
The one possible issue is that reading padding is UB (i think). But here there should be no padding: the struct is a pair number of bytes (alignment 1) and a word (alignment 2), which us aligned due to the previous, and the struct is if alignment 2.
yeah C will be order defined in code, rust will reorder to save space unless you attribute it not to. interesting trivia a blind guy wrote the reordering code. if anyone knows his name please post.
Does the C spec actually specify it needs to be in that order? Been awhile since I messed that that sort of thing. I know in practice they typically just put them in the same order and align on some sort of default packing depending on CPU. Usually to minimize on extra instructions. I have eeked out some extra perf on some CPUs and structs just by moving stuff around and making sure it fits into a cacheline. Sometimes the extra instructions are worth it to unpack some vars if you can get the data into a cacheline. But that depends on your data and usage.
The one possible issue is that reading padding is UB (i think). But here there should be no padding: the struct is a pair number of bytes (alignment 1) and a word (alignment 2), which us aligned due to the previous, and the struct is if alignment 2.