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| author | Matthias Krüger <476013+matthiaskrgr@users.noreply.github.com> | 2025-05-24 16:08:45 +0200 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2025-05-24 16:08:45 +0200 |
| commit | 334d7bd698ca8a6c90824afe42a4c307d0692fea (patch) | |
| tree | a1f6e0aaffb2876e333d3150425d7fd1f9f0bd39 | |
| parent | 3de4f1ccf3873782fae2a3883d029ed3d4542581 (diff) | |
| parent | 0fdeab95250126d135a4bf14cb8959038481204c (diff) | |
| download | rust-334d7bd698ca8a6c90824afe42a4c307d0692fea.tar.gz rust-334d7bd698ca8a6c90824afe42a4c307d0692fea.zip | |
Rollup merge of #137323 - joshlf:transmute-npo, r=RalfJung
Guarantee behavior of transmuting `Option::<T>::None` subject to NPO In https://github.com/rust-lang/rust/pull/115333, we added a guarantee that transmuting from `[0u8; N]` to `Option<P>` is sound where `P` is a pointer type subject to the null pointer optimization (NPO). It would be useful to be able to guarantee the inverse - that a `None::<P>` value can be transmutes to an array and that will yield `[0u8; N]`. Closes #117591
| -rw-r--r-- | library/core/src/option.rs | 30 |
1 files changed, 16 insertions, 14 deletions
diff --git a/library/core/src/option.rs b/library/core/src/option.rs index aed5a043c11..1d264b26076 100644 --- a/library/core/src/option.rs +++ b/library/core/src/option.rs @@ -120,20 +120,22 @@ //! //! Rust guarantees to optimize the following types `T` such that //! [`Option<T>`] has the same size, alignment, and [function call ABI] as `T`. In some -//! of these cases, Rust further guarantees that -//! `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and -//! produces `Option::<T>::None`. These cases are identified by the -//! second column: -//! -//! | `T` | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? | -//! |---------------------------------------------------------------------|----------------------------------------------------------------------| -//! | [`Box<U>`] (specifically, only `Box<U, Global>`) | when `U: Sized` | -//! | `&U` | when `U: Sized` | -//! | `&mut U` | when `U: Sized` | -//! | `fn`, `extern "C" fn`[^extern_fn] | always | -//! | [`num::NonZero*`] | always | -//! | [`ptr::NonNull<U>`] | when `U: Sized` | -//! | `#[repr(transparent)]` struct around one of the types in this list. | when it holds for the inner type | +//! of these cases, Rust further guarantees the following: +//! - `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and produces +//! `Option::<T>::None` +//! - `transmute::<_, [u8; size_of::<T>()]>(Option::<T>::None)` is sound and produces +//! `[0u8; size_of::<T>()]` +//! These cases are identified by the second column: +//! +//! | `T` | Transmuting between `[0u8; size_of::<T>()]` and `Option::<T>::None` sound? | +//! |---------------------------------------------------------------------|----------------------------------------------------------------------------| +//! | [`Box<U>`] (specifically, only `Box<U, Global>`) | when `U: Sized` | +//! | `&U` | when `U: Sized` | +//! | `&mut U` | when `U: Sized` | +//! | `fn`, `extern "C" fn`[^extern_fn] | always | +//! | [`num::NonZero*`] | always | +//! | [`ptr::NonNull<U>`] | when `U: Sized` | +//! | `#[repr(transparent)]` struct around one of the types in this list. | when it holds for the inner type | //! //! [^extern_fn]: this remains true for any argument/return types and any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`) //! |