be more precise about dangling

Author: RalfJung

src/what-unsafe-does.md

@@ -16,7 +16,8 @@ to your program. You definitely *should not* invoke Undefined Behavior.
 Unlike C, Undefined Behavior is pretty limited in scope in Rust. All the core
 language cares about is preventing the following things:
 
-* Dereferencing null, dangling, or unaligned references or raw pointers
+* Loading from or storing to null, dangling, or unaligned references or raw
+  pointers
 * Performing out-of-bounds arithmetic for the computation of an
   `enum`/`struct`/array/slice/tuple field address
 * Reading [uninitialized memory][]
@@ -37,6 +38,10 @@ language cares about is preventing the following things:
 "Producing" a value happens any time a value is assigned, passed to a
 function/primitive operation or returned from a function/primitive operation.
 
+A reference/pointer is "dangling" if not all of the bytes it points to are part
+of the same allocation. The span of bytes it points to is determined by the
+pointer value and the size of the pointee type.
+
 That's it. That's all the causes of Undefined Behavior baked into Rust. Of
 course, unsafe functions and traits are free to declare arbitrary other
 constraints that a program must maintain to avoid Undefined Behavior. For