Address to pointer cast: hint at provenance.

Author: Benjamin Herr

src/expressions/operator-expr.md

@@ -361,9 +361,11 @@ Here `*T` means either `*const T` or `*mut T`.
 * `u8` to `char` cast
     * Casts to the `char` with the corresponding code point.
 * Pointer to address cast
-    * Casting from a valid raw pointer to `usize` will produce a numeric address representing the pointer.
+    * Casting from a valid raw pointer to `usize` will produce the address that is pointed to. Note that the pointer's provenance is lost in this conversion.
 * Address to pointer cast
-    * Casting from `usize` to a raw pointer will produce a pointer with the same value as the original pointer if the `usize` was obtained through a pointer to address cast of a valid pointer of the same type.
+    * Casting from `usize` to a raw pointer will produce a raw pointer to the same location as the original pointer, if the `usize` was obtained through a pointer to address cast of a valid pointer.
+
+      Note that the two pointers are not equivalent. Dereferencing the pointer obtained from the address to pointer cast may be [undefined behavior] if aliasing rules are not followed.
 
 \* if integer-to-float casts with this rounding mode and overflow behavior are
 not supported natively by the hardware, these casts will likely be slower than
@@ -377,6 +379,8 @@ expected.
 number, preferring the one with an even least significant digit if exactly
 halfway between two floating point numbers.
 
+[undefined behavior]: ../behavior-considered-undefined.md
+
 ## Assignment expressions
 
 > **<sup>Syntax</sup>**\