Merge pull request rust-lang#2290 from rust-lang/TC/generalize-wide-pointer-total-size-bound

Generalize the wide-pointer total-size bound

Author: traviscross

src/behavior-considered-undefined.md

@@ -147,7 +147,12 @@ r[undefined.validity.reference-box]
 r[undefined.validity.wide]
 * The [metadata] of a wide reference, [`Box<T>`], or raw pointer must match the type of the [unsized tail]:
   * `dyn Trait` metadata must be a pointer to a compiler-generated vtable for `Trait`. (For raw pointers, this requirement remains a subject of some debate.)
-  * Slice (`[T]`) and `str` metadata must be a valid `usize`. Furthermore, for wide references and [`Box<T>`], this metadata is invalid if it makes the total size of the pointed-to value bigger than `isize::MAX`.
+  * Slice (`[T]`) and `str` metadata must be a valid `usize`.
+
+  In addition, for a wide reference or [`Box<T>`], the metadata is invalid if it makes the total size of the pointed-to value (as determined by `size_of_val`) bigger than `isize::MAX`.
+
+  > [!NOTE]
+  > This bound is on the size of the entire pointed-to value, not just its unsized tail, and it constrains `dyn Trait` metadata just as it does a slice or `str` length. A valid vtable describes an erased type no larger than `isize::MAX`, but a sized prefix can still carry the total past the limit.
 
 r[undefined.validity.valid-range]
 * If a type has a custom range of valid values, then a valid value must be in that range. In the standard library, this affects [`NonNull<T>`] and [`NonZero<T>`].