I believe there is a group working on "safe transmute", which would address the newtype issues... safely. Not just memory safety, but invariant safe, to prevent transmutes in safe rust when the newtype wrapper wants to protect its own invariants.
RFC text: https://github.com/jswrenn/rfcs/blob/safer-transmute/text/00...
I don't know what the status quo is, but it is very exciting work.
There are also crates in this domain that work today. The two I'm aware of are https://docs.rs/zerocopy/latest/zerocopy/ and https://docs.rs/bytemuck/latest/bytemuck/