* Derive macros are now behind a feature flag `derive`.
* `AsBytes` trait has been renamed to `IntoBytes`.
* The
  [Immutable](https://docs.rs/zerocopy/0.8.3/zerocopy/trait.Immutable.html) trait has been added. This marker trait should be added to types that do not have interior mutation (E.g. don't use UnsafeCell or something similar).
* The [KnownLayout](https://docs.rs/zerocopy/0.8.3/zerocopy/trait.KnownLayout.html) trait has been added. This marker trait indicates that zerocopy can reason about certain aspects of a type's layout.
* The return signatures for some zerocopy methods have changed.
* Use `TryFromBytes` for enums that have fewer states than their memory
  layout.

See upgrade guide for more information: google/zerocopy#1680.