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Say that dereferencing a pointer to a ZST is no longer undefined #467

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Merged
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Apr 15, 2025
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Say that dereferencing a pointer to a ZST is no longer undefined #467

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Say that dereferencing a pointer to a ZST is no longer undefined
mattheww Oct 15, 2024
db1fac7
Zero Sized Types: stop saying that "dereferencing" a non-ZST is UB
mattheww Oct 27, 2024
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6 changes: 3 additions & 3 deletions src/exotic-sizes.md
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Expand Up @@ -106,9 +106,9 @@ consequence of types with no size. In particular, pointer offsets are no-ops,
and allocators typically [require a non-zero size][alloc].

Note that references to ZSTs (including empty slices), just like all other
references, must be non-null and suitably aligned. Dereferencing a null or
unaligned pointer to a ZST is [undefined behavior][ub], just like for any other
type.
references, must be non-null and suitably aligned. However, dereferencing a
null pointer to a ZST is not [undefined behavior][ub], unlike pointers to
other types.
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@RalfJung RalfJung Oct 19, 2024 •
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In fact dereferencing (as in, using the * operator on) a null pointer is never UB. addr_of!(*ptr) is always safe, even if ptr is null.

Only non-zero-sized loads, non-zero-sized stores, and field projections at non-zero field offsets have any requirements.

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I've changed it to say "loading or storing through a null pointer to a ZST"


[alloc]: ../std/alloc/trait.GlobalAlloc.html#tymethod.alloc
[ub]: what-unsafe-does.html
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