Merge pull request rust-lang#793 from RalfJung/size-of-val

UB definition: mention size_of_val for determining a reference's span

src/behavior-considered-undefined.md

@@ -61,11 +61,11 @@ code.
 A reference/pointer is "dangling" if it is null or not all of the bytes it
 points to are part of the same allocation (so in particular they all have to be
 part of *some* allocation). The span of bytes it points to is determined by the
-pointer value and the size of the pointee type. As a consequence, if the span is
-empty, "dangling" is the same as "non-null". Note that slices point to their
-entire range, so it is important that the length metadata is never too
-large. In particular, allocations and therefore slices cannot be bigger than
-`isize::MAX` bytes.
+pointer value and the size of the pointee type (using `size_of_val`). As a
+consequence, if the span is empty, "dangling" is the same as "non-null". Note
+that slices point to their entire range, so it is important that the length
+metadata is never too large. In particular, allocations and therefore slices
+cannot be bigger than `isize::MAX` bytes.
 
 > **Note**: Undefined behavior affects the entire program. For example, calling
 > a function in C that exhibits undefined behavior of C means your entire