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Clarify repr(transparent) in other-reprs #329

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Jan 21, 2022
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Clarify repr(transparent) in other-reprs #329

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10fed3a
Clarify repr(transparent) in other-reprs
nico-abram Jan 8, 2022
cd999a7
Update src/other-reprs.md
nico-abram Jan 20, 2022
da24b08
Update src/other-reprs.md
nico-abram Jan 20, 2022
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8 changes: 7 additions & 1 deletion src/other-reprs.md
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Expand Up @@ -62,12 +62,17 @@ whole struct is guaranteed to be the same as that one field.

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
the type it wraps.
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`.

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].

## repr(u*), repr(i*)
Expand Down Expand Up @@ -153,3 +158,4 @@ This is a modifier on `repr(C)` and `repr(Rust)`. It is incompatible with
[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
[no-niche-pull]: https://github.com/rust-lang/rust/pull/68491