I recently proposed Pre-RFC: Trait for deserializing untrusted input, which is similar to some of the arbitrary bits stuff (like the pod crate) discussed here.
One important difference is that we propose having a custom derive. So instead of requiring your fields to be public or requiring you to use an unsafe impl, you can voluntarily do #[derive(ArbitraryBytesSafe)] (that’s the trait name we propose), and the custom derive will verify whether all of your fields, and their composition, are safe for deserializing from arbitrary byte patterns. It works for private fields too, but because you have to explicitly use the derive, it won’t just automatically apply to your type, invalidating your invariants.