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Fields must fit in the type, even for repr(Rust) #2166

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Fields must fit in the type, even for repr(Rust) #2166

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be8acd0
Fields must fit in the struct
Darksonn Feb 11, 2026
47a5caf
Restrict guarantees to constructible fields
Darksonn Mar 11, 2026
b1f9d05
Make it more clear that uninhabited exception is only for enums
Darksonn Mar 11, 2026
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8 changes: 7 additions & 1 deletion src/type-layout.md
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Expand Up @@ -168,12 +168,18 @@ The only data layout guarantees made by this representation are those required f

1. The offset of a field is divisible by that field's alignment.
2. The alignment of the type is at least the maximum alignment of its fields.
3. For any field, its offset plus its size is at most the size of the type.

For enums, the above guarantees only apply to fields of inhabited variants.

r[layout.repr.rust.layout.struct]
For [structs], it is further guaranteed that the fields do not overlap. That is, the fields can be ordered such that the offset plus the size of any field is less than or equal to the offset of the next field in the ordering. The ordering does not have to be the same as the order in which the fields are specified in the declaration of the type.
For [structs], it is further guaranteed that the fields do not overlap. That is, the fields can be ordered such that the offset plus the size of any field is less than or equal to the offset of the next field in the ordering, or for the last field, the size of the struct. The ordering does not have to be the same as the order in which the fields are specified in the declaration of the type.

Be aware that this guarantee does not imply that the fields have distinct addresses: zero-sized types may have the same address as other fields in the same struct.

r[layout.repr.rust.layout.enum]
For enums, a variant of the enum is said to be inhabited if all of its fields are [inhabited](glossary.html#inhabited). When an enum variant is inhabited, its fields are laid out according to the same guarantees as a struct. Otherwise, nothing is guaranteed about the fields. This allows the enum to be smaller than fields of uninhabited variants.

r[layout.repr.rust.unspecified]
There are no other guarantees of data layout made by this representation.

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