> Or, more precisely, reserves the right to do so in a future version. rustc does not currently do this

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]