If you're writing a kernel in Rust, you presumably aren't guaranteeing that all the programs your kernel runs are also written in Rust. To that end, being able to randomize fields sounds plausibly useful.

However, I would imagine it's probably better done by writing a custom item decorator that randomizes the field order (and places whatever `#[repr()]` attribute is necessary to tell the compiler to use the declaration order). Which is to say, the kernel author can write the necessary extension, Rust doesn't need to provide it.