> Does Rust fundamentally guarantee that if you make a struct, its fields will lay out in memory in the order that you defined them? Can it be used to interact with APIs (really ABIs) who expect a C struct (or pointer to one)?
You have to specify this behavior with #[repr(C)]. Otherwise, the compiler will rearrange fields to try to optimize packing and alignment.
Or, more precisely, reserves the right to do so in a future version. rustc does not currently do this. There are some compiler flags to randomize the struct layout just to make sure that you’re not implicitly depending on the order.
You have to specify this behavior with #[repr(C)]. Otherwise, the compiler will rearrange fields to try to optimize packing and alignment.