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Generalize the wide-pointer total-size bound #2290

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traviscross merged 1 commit into from
Jun 16, 2026
Merged

Generalize the wide-pointer total-size bound #2290

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7 changes: 6 additions & 1 deletion src/behavior-considered-undefined.md
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Expand Up @@ -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`.

@RalfJung RalfJung Jun 29, 2026 •
edited by rustbot
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FWIW I would consider all of this a derived property. It follows from the fact that wide references and Box must be dereferenceable for the actual dynamic size given by their pointee type and metadata, and it is impossible to be dereferenceable for more than isize::MAX bytes as that is the largest an allocation can be.

So, if this is explicitly stated at all, IMO it should only be done non-normatively, as the normative part should avoid such redundancies.

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> [!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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