https://doc.rust-lang.org/reference/type-layout.html
https://github.com/rust-lang/compiler-team/issues/457
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]
https://doc.rust-lang.org/reference/type-layout.html
https://github.com/rust-lang/compiler-team/issues/457