Add memory layout documentation to generic NonZero<T>

Author: carlsverre

library/core/src/num/nonzero.rs

@@ -90,6 +90,26 @@ impl_zeroable_primitive!(
 ///
 /// assert_eq!(size_of::<Option<NonZero<u32>>>(), size_of::<u32>());
 /// ```
+///
+/// # Layout
+///
+/// `NonZero<T>` is guaranteed to have the same layout and bit validity as `T`
+/// with the exception that the all-zero bit pattern is not a valid instance.
+/// `Option<NonZero<T>>` is guaranteed to be compatible with `T`, including in
+/// FFI.
+///
+/// Thanks to the [null pointer optimization], `NonZero<T>` and
+/// `Option<NonZero<T>>` are guaranteed to have the same size and alignment:
+///
+/// ```
+/// # use std::mem::{size_of, align_of};
+/// use std::num::NonZero;
+///
+/// assert_eq!(size_of::<NonZero<u32>>(), size_of::<Option<NonZero<u32>>>());
+/// assert_eq!(align_of::<NonZero<u32>>(), align_of::<Option<NonZero<u32>>>());
+/// ```
+///
+/// [null pointer optimization]: crate::option#representation
 #[stable(feature = "generic_nonzero", since = "1.79.0")]
 #[repr(transparent)]
 #[rustc_nonnull_optimization_guaranteed]