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Document the enum changes in RFC 2195 #879

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Sep 4, 2020
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Document the enum changes in RFC 2195 #879

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8a76095
Document the enum changes in RFC 2195
poliorcetics Aug 21, 2020
fddf097
Address review
poliorcetics Aug 22, 2020
5a46f81
Fix nits from review, move a paragraph to a better place
poliorcetics Aug 24, 2020
ddb2fe9
Add a link and reword the start of a paragraph
poliorcetics Aug 25, 2020
75708a2
Address concerns from review and add a subsection about combining pri…
poliorcetics Aug 26, 2020
4d133a0
Improve the section Combining primitive representations of enums with…
poliorcetics Aug 27, 2020
6808c5e
Fix nits
poliorcetics Aug 31, 2020
73eae8e
Fix nits 2
poliorcetics Sep 1, 2020
6e8d93a
Remove the note altogether
poliorcetics Sep 1, 2020
eb4ec25
Use 'representation' instead of 'layout' in the examples
poliorcetics Sep 2, 2020
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37 changes: 16 additions & 21 deletions src/type-layout.md
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Expand Up @@ -361,20 +361,6 @@ struct MyDFields;

> Note: `union`s with non-`Copy` fields are unstable, see [55149].

<span id="c-primitive-representation">Combining the `repr(C)`
and a primitive representation is only defined for enums with fields. The
primitive representation modifies the `repr(C)` by changing the representation
of the discriminant enum to have the representation of the chosen primitive
representation. So, if you chose the `u8` representation, then the discriminant
enum would have a size and alignment of 1 byte.</span>

> Note: This representation is primarily intended for Rust code that wants to
> interoperate with the idioms of preexisting C/C++ codebases. If you have
> control over both the Rust and C code, such as using C as FFI glue between
> Rust and some third language, then you should use a
> [primitive representation](#primitive-representation-of-enums-with-fields)
> instead.

### Primitive representations

The *primitive representations* are the representations with the same names as
Expand All @@ -383,12 +369,22 @@ the primitive integer types. That is: `u8`, `u16`, `u32`, `u64`, `u128`,

Primitive representations can only be applied to enumerations and have
different behavior whether the enum has fields or no fields. It is an error
for [zero-variant enumerations] to have a primitive representation.
for [zero-variant enumerations] to have a primitive representation. Combining
two primitive representations together is an error.

Combining two primitive representations together is unspecified.
<span id="c-primitive-representation">Combining the `repr(C)`
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and a primitive representation is only defined for enums with fields. The
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primitive representation modifies the `repr(C)` by changing the representation
of the discriminant enum to have the representation of the chosen primitive
representation. So, if you chose the `u8` representation, then the discriminant
enum would have a size and alignment of 1 byte.</span>

Combining the `C` representation and a primitive representation is described
[above](#c-primitive-representation).
> Note: Primitive representations are primarily intended for Rust code that
> wants to interoperate with the idioms of preexisting C/C++ codebases. If you
> have control over both the Rust and C code, such as using C as FFI glue
> between Rust and some third language, then you should use a
> [primitive representation](#primitive-representation-of-enums-with-fields)
> instead.
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#### Primitive Representation of Field-less Enums

Expand All @@ -408,9 +404,6 @@ and the remaining fields are the fields of that variant.
> to follow The Exact Word Of The Standard the tag member should be wrapped in
> a `struct`).

Because unions with non-`Copy` fields aren't allowed, this representation can
only be expressed in Rust if every field is also [`Copy`].

```rust
// This enum has the same layout as ...
#[repr(u8)]
Expand Down Expand Up @@ -452,6 +445,8 @@ struct MyVariantC { tag: MyEnumDiscriminant, x: u32, y: u8 }
struct MyVariantD(MyEnumDiscriminant);
```

> Note: `union`s with non-`Copy` fields are unstable, see [55149].

### The alignment modifiers

The `align` and `packed` modifiers can be used to respectively raise or lower
Expand Down