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Update section on default layout for repr(Rust) #1396

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Aug 26, 2023
Merged

Update section on default layout for repr(Rust) #1396

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c6d4744
Update `type-layout.md` for `repr(Rust)`
Centri3 Aug 17, 2023
9f9e7a7
Add `the-default-representation` anchor
Centri3 Aug 17, 2023
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16 changes: 9 additions & 7 deletions src/type-layout.md
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Expand Up @@ -88,7 +88,7 @@ String slices are a UTF-8 representation of characters that have the same layout

## Tuple Layout

Tuples are laid out according to the [default representation][Default].
Tuples are laid out according to the [`Rust` representation][`Rust`].

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.
Expand All @@ -110,7 +110,7 @@ All user-defined composite types (`struct`s, `enum`s, and `union`s) have a
*representation* that specifies what the layout is for the type. The possible
representations for a type are:

- [Default]
- [`Rust`] (default)
- [`C`]
- The [primitive representations]
- [`transparent`]
Expand Down Expand Up @@ -159,10 +159,12 @@ not change the layout of the fields themselves. For example, a struct with a
`C` representation that contains a struct `Inner` with the default
representation will not change the layout of `Inner`.

### The Default Representation
### <a id="the-default-representation"></a> The `Rust` Representation

Nominal types without a `repr` attribute have the default representation.
Informally, this representation is also called the `rust` representation.
The `Rust` representation is the default representation for nominal types
without a `repr` attribute. Using this representation explicitly through a
`repr` attribute is guaranteed to be the same as omitting the attribute
entirely.

The only data layout guarantees made by this representation are those required
for soundness. They are:
Expand Down Expand Up @@ -543,7 +545,7 @@ important consequence of these rules is that a type with `#[repr(packed(1))]`

The `align` and `packed` modifiers cannot be applied on the same type and a
`packed` type cannot transitively contain another `align`ed type. `align` and
`packed` may only be applied to the [default] and [`C`] representations.
`packed` may only be applied to the [`Rust`] and [`C`] representations.

The `align` modifier can also be applied on an `enum`.
When it is, the effect on the `enum`'s alignment is the same as if the `enum`
Expand Down Expand Up @@ -604,7 +606,7 @@ used with any other representation.
[undefined behavior]: behavior-considered-undefined.md
[55149]: https://github.com/rust-lang/rust/issues/55149
[`PhantomData<T>`]: special-types-and-traits.md#phantomdatat
[Default]: #the-default-representation
[`Rust`]: #the-rust-representation
[`C`]: #the-c-representation
[primitive representations]: #primitive-representations
[structs]: items/structs.md
Expand Down