NonNull, NonZero*

Author: RalfJung

src/what-unsafe-does.md

@@ -28,6 +28,7 @@ language cares about is preventing the following things:
     * an undefined `enum` discriminant
     * a `char` outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
     * a non-utf8 `str`
+    * a `NonNull` or `NonZero*` that is 0
     * a compound type (`enum`/`struct`/array/tuple) with an invalid field
 * Unwinding into another language
 * Causing a [data race][race]