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.
Does not currently do what ?
Rust certainly will re-arrange the layout of a default repr(Rust) struct to make it smaller, for example: https://rust.godbolt.org/z/7KsqvnE9o
[Edited to provide a nicer Godbolt example which compares the two layout strategies]