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dangling pointers: turn some consequences of the definition into notes #2336

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Merged
traviscross merged 2 commits into from
Sep 1, 2026
Merged

dangling pointers: turn some consequences of the definition into notes #2336

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dangling pointers: turn some consequences of the definition into notes
RalfJung Aug 23, 2026
42163cf
Use an intra-doc path for the allocation link
traviscross Sep 1, 2026
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14 changes: 6 additions & 8 deletions src/behavior-considered-undefined.md
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Expand Up @@ -100,16 +100,13 @@ r[undefined.dangling]
[dangling]: #dangling-pointers

r[undefined.dangling.def]
A reference/pointer is "dangling" if not all of the bytes it [points to] are part of the same live allocation (so in particular they all have to be part of *some* allocation).
A reference/pointer is "dangling" if not all of the bytes it [points to] are part of the same live [allocation] (so in particular they all have to be part of *some* allocation).

r[undefined.dangling.zero-size]
If the [size is 0][zero-sized], then the pointer is trivially never "dangling" (even if it is a null pointer).

r[undefined.dangling.dynamic-size]
Note that dynamically sized types (such as slices and strings) point to their entire range, so it is important that the length [metadata] is never too large.
> [!NOTE]
> This implies that the dynamic size of a Rust value (as determined by `size_of_val`) must never exceed `isize::MAX`, since it is impossible for a single allocation to be larger than `isize::MAX`.

r[undefined.dangling.alloc-limit]
In particular, the dynamic size of a Rust value (as determined by `size_of_val`) must never exceed `isize::MAX`, since it is impossible for a single allocation to be larger than `isize::MAX`.
> [!NOTE]
> This also implies that if the [size is 0][zero-sized], then the pointer is trivially never "dangling" (even if it is a null pointer).

r[undefined.validity]
## Invalid values
Expand Down Expand Up @@ -201,6 +198,7 @@ r[undefined.validity.undef]
[`bool`]: types/boolean.md
[`const`]: items/constant-items.md
[abi]: items/external-blocks.md#abi
[allocation]: core::ptr#allocation

@RalfJung RalfJung Sep 1, 2026

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Oh I had no idea these work here, nice :)

[const contexts]: const-eval.const-context
[`target_feature`]: attributes/codegen.md#the-target_feature-attribute
[`UnsafeCell<U>`]: std::cell::UnsafeCell
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