clarify rules for ZST Boxes

library/core/src/ptr/mod.rs

@@ -16,12 +16,15 @@
 //! provided at this point are very minimal:
 //!
 //! * A [null] pointer is *never* valid, not even for accesses of [size zero][zst].
-//! * All pointers (except for the null pointer) are valid for all operations of
-//!   [size zero][zst].
 //! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer
 //!   be *dereferenceable*: the memory range of the given size starting at the pointer must all be
 //!   within the bounds of a single allocated object. Note that in Rust,
 //!   every (stack-allocated) variable is considered a separate allocated object.
+//! * Even for operations of [size zero][zst], the pointer must not be "dangling" in the sense of
+//!   pointing to deallocated memory. However, casting any non-zero integer to a pointer is valid
+//!   for zero-sized accesses. This corresponds to writing your own allocator; allocating zero-sized
+//!   objects is not very hard. In contrast, when you use the standard allocator, after memory got
+//!   deallocated, even zero-sized accesses to that memory are invalid.
 //! * All accesses performed by functions in this module are *non-atomic* in the sense
 //!   of [atomic operations] used to synchronize between threads. This means it is
 //!   undefined behavior to perform two concurrent accesses to the same location from different