For anyone wondering, NonNull (along with other "constrained" types like NonZero integers) uses internal compiler attributes (https://doc.rust-lang.org/src/core/ptr/non_null.rs.html#72):

  #[rustc_layout_scalar_valid_range_start(1)]

This gives rustc enough information to perform niche optimizations such as collapsing Option<NonNull<T>>.

You can technically use those for your own types, but it requires nightly, obviously.