Explicitly note any cast not defined in the documentation is illegal.

Author: mcclure

src/expressions/operator-expr.md

@@ -329,6 +329,8 @@ Here `*T` means either `*const T` or `*mut T`.
 
 \*\* only for closures that do not capture (close over) any local variables
 
+Any cast that does not fit either a coercion rule or an entry in the above table is a compiler error.
+
 ### Semantics
 
 * Numeric cast