From ca00ee8314cf9e3e26defd81c4d08a8212e41350 Mon Sep 17 00:00:00 2001 From: Nicolas Abram Date: Thu, 20 Jan 2022 23:14:16 -0300 Subject: [PATCH] Clarify repr(transparent) in other-reprs (#329) Co-authored-by: Yuki Okushi --- src/other-reprs.md | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/src/other-reprs.md b/src/other-reprs.md index fe06faf..93da729 100644 --- a/src/other-reprs.md +++ b/src/other-reprs.md @@ -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*) @@ -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