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| author | bors <bors@rust-lang.org> | 2019-06-19 06:49:13 +0000 |
|---|---|---|
| committer | bors <bors@rust-lang.org> | 2019-06-19 06:49:13 +0000 |
| commit | a6cbf2d1344418cd2807cc5380ef1247647a1e12 (patch) | |
| tree | cd2c5efba7fae486762161de293c684b1d66d962 /src/libcore | |
| parent | 605ea9d05c48957a291eec11eb7339788c3140ed (diff) | |
| parent | fde341a4ef6a5728dfd1acb5de0b238918a2dd44 (diff) | |
| download | rust-a6cbf2d1344418cd2807cc5380ef1247647a1e12.tar.gz rust-a6cbf2d1344418cd2807cc5380ef1247647a1e12.zip | |
Auto merge of #61945 - Centril:rollup-xdqo2mn, r=Centril
Rollup of 11 pull requests Successful merges: - #61505 (Only show methods that appear in `impl` blocks in the Implementors sections of trait doc pages) - #61701 (move stray run-pass const tests into const/ folder) - #61748 (Tweak transparent enums and unions diagnostic spans) - #61802 (Make MaybeUninit #[repr(transparent)]) - #61839 (ci: Add a script for generating CPU usage graphs) - #61842 (Remove unnecessary lift calls) - #61843 (Turn down the myriad-closures test) - #61896 (rustc_typeck: correctly compute `Substs` for `Res::SelfCtor`.) - #61898 (syntax: Factor out common fields from `SyntaxExtension` variants) - #61938 (create an issue for miri even in status test-fail) - #61941 (Preserve generator and yield source for error messages) Failed merges: r? @ghost
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>, |