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reference metadata validity: the isize::MAX bound is a consequence #2352

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reference metadata validity: the isize::MAX bound is a consequence #2352

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9 changes: 4 additions & 5 deletions src/behavior-considered-undefined.md
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Expand Up @@ -147,16 +147,15 @@ r[undefined.validity.union]
r[undefined.validity.reference-box]
* A reference or [`Box<T>`] must be aligned and non-null, it cannot be [dangling], and it must point to a valid value (in case of dynamically sized types, using the actual dynamic type of the pointee as determined by the [metadata]). Note that the last point (about pointing to a valid value) remains a subject of some debate.

> [!NOTE]
> The non-[dangling] requirement implies that the total size of the [pointed-to][points to] value (fixed-size prefix plus [unsized tail]) must be no bigger than `isize::MAX`.
> 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.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`.

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>`].

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