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Document transparent representation #389

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Aug 20, 2018
Merged

Document transparent representation #389

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18 changes: 18 additions & 0 deletions src/type-layout.md
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Expand Up @@ -302,6 +302,23 @@ Like all ways to create undefined behavior in safe Rust, this is a bug.

</div>

### The `transparent` Representation

The `transparent` representation can only be used on `struct`s that have a
single non-zero sized field and any number of zero-sized fields, including
[`PhantomData<T>`].

Structs with this representation have the same layout and ABI as the single
non-zero sized field.

This is different than the `C` representation because
a struct with the `C` representation will always have the ABI of a `C` `struct`
while, for example, a struct with the `transparent` representation with a
primitive field will have the ABI of the primitive field.

Because this representation delegates type layout to another type, it cannot be
used with any other representation.

[`align_of_val`]: ../std/mem/fn.align_of_val.html
[`size_of_val`]: ../std/mem/fn.size_of_val.html
[`align_of`]: ../std/mem/fn.align_of.html
Expand All @@ -312,3 +329,4 @@ Like all ways to create undefined behavior in safe Rust, this is a bug.
[zero-variant enumerations]: items/enumerations.html#zero-variant-enums
[undefined behavior]: behavior-considered-undefined.html
[27060]: https://github.com/rust-lang/rust/issues/27060
[`PhantomData<T>`]: special-types-and-traits.html#phantomdatat