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Specify guarantees for repr(rust) structs #1152

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merged 7 commits into from
Sep 19, 2022
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Specify guarantees for repr(rust) structs #1152

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5a8a664
Specify guarantees for repr(rust) structs
Darksonn Jan 31, 2022
e9a1ddf
Address review
Darksonn Feb 6, 2022
377c524
Fix typo
Darksonn Mar 22, 2022
f9ec1d2
Be less struct-centric
Darksonn May 9, 2022
1275bcc
Rephrase layout of tuples
Darksonn May 9, 2022
3ecc681
Remove stray 'minimum'
Darksonn May 9, 2022
1ae7c2d
Link to default representation.
ehuss Sep 19, 2022
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24 changes: 21 additions & 3 deletions src/type-layout.md
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Expand Up @@ -86,9 +86,9 @@ String slices are a UTF-8 representation of characters that have the same layout

## Tuple Layout

Tuples do not have any guarantees about their layout.
Tuples are laid out according to the [default representation][Default].

The exception to this is the unit tuple (`()`) which is guaranteed as a
The exception to this is the unit tuple (`()`), which is guaranteed as a
zero-sized type to have a size of 0 and an alignment of 1.
Comment on lines +91 to 92
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Does the former statement imply this statement?

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Not as I wrote it here, but you could certainly argue that it should be changed so it does.


## Trait Object Layout
Expand Down Expand Up @@ -162,7 +162,25 @@ representation will not change the layout of `Inner`.
Nominal types without a `repr` attribute have the default representation.
Informally, this representation is also called the `rust` representation.

There are no guarantees of data layout made by this representation.
The only data layout guarantees made by this representation are those required
for soundness. They are:

1. The fields are properly aligned.
2. The fields do not overlap.
3. The alignment of the type is at least the maximum alignment of its fields.

Formally, the first guarantee means that the offset of any field is divisible by
that field's alignment. The second guarantee means that 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.

Be aware that the second 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.

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There are no other guarantees of data layout made by this representation.

### The `C` Representation

Expand Down