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Update the repr(transparent) section to reflect the current state #376

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Aug 15, 2022
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Update the repr(transparent) section to reflect the current state #376

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Update the `repr(transparent)` section to reflect the current state
JohnTitor Jul 27, 2022
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JohnTitor Aug 15, 2022
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21 changes: 13 additions & 8 deletions src/other-reprs.md
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Expand Up @@ -56,24 +56,27 @@ compiled as normal.)

## repr(transparent)

This can only be used on structs with a single non-zero-sized field (there may
be additional zero-sized fields). The effect is that the layout and ABI of the
whole struct is guaranteed to be the same as that one field.
This can only be used on structs with a single non-zero-sized field
or enums with only one non-zero-sized field (there may be additional zero-sized fields).
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The effect is that the layout and ABI of the whole struct/enum is guaranteed to be the same as that one field.

> NOTE: There's a `transparent_unions` nightly feature to apply `repr(transparent)` to unions,
> but it hasn't been stabilized due to design concerns. See the [tracking issue][issue-60405] for more details.

The goal is to make it possible to transmute between the single field and the
struct. An example of that is [`UnsafeCell`], which can be transmuted into
struct/enum. An example of that is [`UnsafeCell`], which can be transmuted into
the type it wraps ([`UnsafeCell`] also uses the unstable [no_niche][no-niche-pull],
so its ABI is not actually guaranteed to be the same when nested in other types).

Also, passing the struct through FFI where the inner field type is expected on
the other side is guaranteed to work. In particular, this is necessary for `struct
Foo(f32)` to always have the same ABI as `f32`.
Also, passing the struct/enum through FFI where the inner field type is expected on
the other side is guaranteed to work. In particular, this is necessary for
`struct Foo(f32)` or `enum Foo { Bar(f32) }` to always have the same ABI as `f32`.

This repr is only considered part of the public ABI of a type if either the single
field is `pub`, or if its layout is documented in prose. Otherwise, the layout should
not be relied upon by other crates.

More details are in the [RFC][rfc-transparent].
More details are in the [RFC 1758][rfc-transparent] and the [RFC 2645][rfc-transparent-unions-enums].

## repr(u*), repr(i*)

Expand Down Expand Up @@ -153,8 +156,10 @@ This is a modifier on `repr(C)` and `repr(Rust)`. It is incompatible with
[unsafe code guidelines]: https://rust-lang.github.io/unsafe-code-guidelines/layout.html
[drop flags]: drop-flags.html
[ub loads]: https://github.com/rust-lang/rust/issues/27060
[issue-60405]: https://github.com/rust-lang/rust/issues/60405
[`UnsafeCell`]: ../std/cell/struct.UnsafeCell.html
[rfc-transparent]: https://github.com/rust-lang/rfcs/blob/master/text/1758-repr-transparent.md
[rfc-transparent-unions-enums]: https://rust-lang.github.io/rfcs/2645-transparent-unions.html
[really-tagged]: https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md
[rust-bindgen]: https://rust-lang.github.io/rust-bindgen/
[cbindgen]: https://github.com/eqrion/cbindgen
Expand Down