Rust will reorder struct members in the most efficient way possible unless you explicitly tell it not to. This is also how most other low level languages (like

Rust will reorder struct members in the most efficient way possible unless you explicitly tell it not to. This is also how most other low level languages (like C++) do it. The main reason to have an option at all is it makes it easier to work with arrays of bytes. For example, in networking if you receive a packet, instead of parsing a packet you can just cast it to a struct with fields in the correct order. And if you send a packet, you can just cast the struct to a byte array. This avoids a lot of copying and can greatly simplify code.