Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Specify guarantees for repr(rust) structs #1152

Merged
merged 7 commits into from
Sep 19, 2022
Merged

Specify guarantees for repr(rust) structs #1152

Changes from 6 commits
Commits
Show all changes
7 commits
Select commit Hold shift + click to select a range
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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
24 changes: 21 additions & 3 deletions src/type-layout.md
View file
Open in desktop
Original file line number Diff line number Diff line change
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.
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

What does "the default representation" refer to here?

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The section with most of my changes has the heading "The Default Representation", and that's what I'm referring to. I don't know if there's a better way to phrase it, or perhaps add a link to the heading?

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

+1 a link to the heading would be great, and with an explicit note that they may not be the same representation as a struct with equivalent fields.


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
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Does the former statement imply this statement?

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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.

Darksonn marked this conversation as resolved.
Show resolved Hide resolved
There are no other guarantees of data layout made by this representation.

### The `C` Representation

Expand Down