Add `zerocopy`-aware methods

[kupiakos]

Gated by a non-default feature.

• <T: FromBytes + ?Sized>(&mut T) -> Out<[u8]>
  • AsBytes is not required as it is for &mut T -> &mut [u8] since the Out does not expose uninitialized bytes
• <T: FromBytes + ?Sized>(Out<T>) -> Out<[u8]>
  • mem::size_of_val should work here?

[kupiakos]

These could also work, but there are likely conflicting blanket impls for these to all be From methods:

• <T: ?Sized + AsMaybeUninit>(&MaybeUninit<T>) -> &[MaybeUninit<u8>]
• <T: ?Sized + AsMaybeUninit>(&mut MaybeUninit<T>) -> &mut [MaybeUninit<u8>]
• <T: ?Sized + AsMaybeUninit>(&mut MaybeUninit<T>) -> Out<[u8]>
  • You can put anything into a MaybeUninit, and no uninit bytes are exposed by the Out. This is the mutable version of the above.
• <T: ?Sized, U>(&mut MaybeUninit<T>) -> Option<Out<U>>
• <T: FromBytes + ?Sized, U: AsBytes + FromBytes>(Out<T>) -> Option<Out<U>>
  • The U: AsBytes makes it so writeing a U cannot write uninit that is invalid for T
  • Fallible since T could be unaligned for U
• <T: FromBytes + ?Sized, U: AsBytes + FromBytes>(Out<T>) -> Option<Out<U>>
  • The U: AsBytes makes it so writeing a U cannot write uninit that is invalid for T
  • Fallible since T could be unaligned for U
• <T: FromBytes + ?Sized, U: AsBytes + FromBytes + Unaligned>(Out<T>) -> Out<U>
  • The U: AsBytes makes it so writeing a U cannot write uninit that is invalid for T
  • For this to be infallible, U cannot have a stricter alignment requirement than T

[kupiakos]

The most flexible and simple of these is the Out<T: ?Sized + FromBytes> -> Out<[u8]> conversion. The rest either involve Unaligned bounds or dynamic alignment checks.