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Add alignment to the NPO guarantee #114845

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Sep 2, 2023
Merged

Add alignment to the NPO guarantee #114845

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14 changes: 14 additions & 0 deletions library/core/src/num/nonzero.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,20 @@ macro_rules! nonzero_integers {
/// with the exception that `0` is not a valid instance.
#[doc = concat!("`Option<", stringify!($Ty), ">` is guaranteed to be compatible with `", stringify!($Int), "`,")]
/// including in FFI.
///
/// Thanks to the [null pointer optimization],
#[doc = concat!("`", stringify!($Ty), "` and `Option<", stringify!($Ty), ">`")]
/// are guaranteed to have the same size and alignment:
///
/// ```
/// # use std::mem::{size_of, align_of};
#[doc = concat!("use std::num::", stringify!($Ty), ";")]
///
#[doc = concat!("assert_eq!(size_of::<", stringify!($Ty), ">(), size_of::<Option<", stringify!($Ty), ">>());")]
#[doc = concat!("assert_eq!(align_of::<", stringify!($Ty), ">(), align_of::<Option<", stringify!($Ty), ">>());")]
/// ```
///
/// [null pointer optimization]: crate::option#representation
#[$stability]
#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
#[repr(transparent)]
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2 changes: 1 addition & 1 deletion library/core/src/option.rs
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Expand Up @@ -119,7 +119,7 @@
//! # Representation
//!
//! Rust guarantees to optimize the following types `T` such that
//! [`Option<T>`] has the same size as `T`:
//! [`Option<T>`] has the same size and alignment as `T`:
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These two words are the core of the change; the other places refer back to here.

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We actually also need them to be function-call-ABI compatible, right? Even types with the same size and alignment can have different ABIs.

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That's already guaranteed for a few types, like https://doc.rust-lang.org/stable/std/num/struct.NonZeroU32.html#layout-1.

That said, I'd like to keep this PR+FCP to just the hopefully-obvious and non-controversial addition of alignment, and something else can add more.

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Hm I see. We should probably standardize terminology at some point, "FFI" isn't really clear that it talks about ABI -- after all, for by-ref FFI, you don't need ABI compatibility.

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And avoiding deciding on the terminology for this is part of why I'm happy keeping this to just something we have a word for already 🙃

//!
//! * [`Box<U>`]
//! * `&U`
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18 changes: 18 additions & 0 deletions library/core/src/ptr/non_null.rs
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Expand Up @@ -43,9 +43,27 @@ use crate::slice::{self, SliceIndex};
/// it is your responsibility to ensure that `as_mut` is never called, and `as_ptr`
/// is never used for mutation.
///
/// # Representation
///
/// Thanks to the [null pointer optimization],
/// `NonNull<T>` and `Option<NonNull<T>>`
/// are guaranteed to have the same size and alignment:
///
/// ```
/// # use std::mem::{size_of, align_of};
/// use std::ptr::NonNull;
///
/// assert_eq!(size_of::<NonNull<i16>>(), size_of::<Option<NonNull<i16>>>());
/// assert_eq!(align_of::<NonNull<i16>>(), align_of::<Option<NonNull<i16>>>());
///
/// assert_eq!(size_of::<NonNull<str>>(), size_of::<Option<NonNull<str>>>());
/// assert_eq!(align_of::<NonNull<str>>(), align_of::<Option<NonNull<str>>>());
/// ```
///
/// [covariant]: https://doc.rust-lang.org/reference/subtyping.html
/// [`PhantomData`]: crate::marker::PhantomData
/// [`UnsafeCell<T>`]: crate::cell::UnsafeCell
/// [null pointer optimization]: crate::option#representation
#[stable(feature = "nonnull", since = "1.25.0")]
#[repr(transparent)]
#[rustc_layout_scalar_valid_range_start(1)]
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