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| author | Mazdak Farrokhzad <twingoow@gmail.com> | 2019-06-19 01:52:03 +0200 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2019-06-19 01:52:03 +0200 |
| commit | e416932bab05b70da73f0d7d9f6b751c433324fd (patch) | |
| tree | 856de4eacb7c85a76e090e4241745709483f740f /src/libcore | |
| parent | 06d2a891c72fa29477114d67c8672e0eaacddd0b (diff) | |
| parent | 0f9dc6c48e51637a5d54fc791df94d104c1e0aee (diff) | |
| download | rust-e416932bab05b70da73f0d7d9f6b751c433324fd.tar.gz rust-e416932bab05b70da73f0d7d9f6b751c433324fd.zip | |
Rollup merge of #61802 - mjbshaw:maybe-uninit-transparent, r=cramertj
Make MaybeUninit #[repr(transparent)] Tracking issue: #60405
Diffstat (limited to 'src/libcore')
| -rw-r--r-- | src/libcore/lib.rs | 1 | ||||
| -rw-r--r-- | src/libcore/mem/maybe_uninit.rs | 16 |
2 files changed, 16 insertions, 1 deletions
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs index 030f4f1d12c..1bfb852424d 100644 --- a/src/libcore/lib.rs +++ b/src/libcore/lib.rs @@ -100,6 +100,7 @@ #![feature(staged_api)] #![feature(std_internals)] #![feature(stmt_expr_attributes)] +#![cfg_attr(not(bootstrap), feature(transparent_unions))] #![feature(unboxed_closures)] #![feature(unsized_locals)] #![feature(untagged_unions)] diff --git a/src/libcore/mem/maybe_uninit.rs b/src/libcore/mem/maybe_uninit.rs index eeff9d0303a..28e1e22ba7f 100644 --- a/src/libcore/mem/maybe_uninit.rs +++ b/src/libcore/mem/maybe_uninit.rs @@ -172,7 +172,7 @@ use crate::mem::ManuallyDrop; /// /// # Layout /// -/// `MaybeUninit<T>` is guaranteed to have the same size and alignment as `T`: +/// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`: /// /// ```rust /// use std::mem::{MaybeUninit, size_of, align_of}; @@ -191,9 +191,23 @@ use crate::mem::ManuallyDrop; /// assert_eq!(size_of::<Option<bool>>(), 1); /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2); /// ``` +/// +/// If `T` is FFI-safe, then so is `MaybeUninit<T>`. +/// +/// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size, +/// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and +/// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type +/// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`. +/// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the +/// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact +/// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not +/// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has +/// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that +/// guarantee may evolve. #[allow(missing_debug_implementations)] #[stable(feature = "maybe_uninit", since = "1.36.0")] #[derive(Copy)] +#[cfg_attr(not(bootstrap), repr(transparent))] pub union MaybeUninit<T> { uninit: (), value: ManuallyDrop<T>, |