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Rename SizeEq to SizeCompat, make it safe #2872

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Rename SizeEq to SizeCompat, make it safe #2872

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30 changes: 9 additions & 21 deletions src/impls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -112,7 +112,7 @@ const _: () = unsafe {
*byte.unaligned_as_ref() < 2
})
};
impl_size_eq!(bool, u8);
impl_size_compat!(bool, u8);

// SAFETY:
// - `Immutable`: `char` self-evidently does not contain any `UnsafeCell`s.
Expand Down Expand Up @@ -144,7 +144,7 @@ const _: () = unsafe {
});
};

impl_size_eq!(char, Unalign<u32>);
impl_size_compat!(char, Unalign<u32>);

// SAFETY: Per the Reference [1], `str` has the same layout as `[u8]`.
// - `Immutable`: `[u8]` does not contain any `UnsafeCell`s.
Expand Down Expand Up @@ -179,13 +179,13 @@ const _: () = unsafe {
})
};

impl_size_eq!(str, [u8]);
impl_size_compat!(str, [u8]);

macro_rules! unsafe_impl_try_from_bytes_for_nonzero {
($($nonzero:ident[$prim:ty]),*) => {
$(
unsafe_impl!(=> TryFromBytes for $nonzero; |n| {
impl_size_eq!($nonzero, Unalign<$prim>);
impl_size_compat!($nonzero, Unalign<$prim>);

let n = n.transmute::<Unalign<$prim>, invariant::Valid, _>();
$nonzero::new(n.read_unaligned().into_inner()).is_some()
Expand Down Expand Up @@ -426,7 +426,7 @@ mod atomics {
crate::util::macros::__unsafe();

use core::cell::UnsafeCell;
use crate::pointer::{SizeEq, TransmuteFrom, invariant::Valid};
use crate::pointer::{SizeCompat, TransmuteFrom, invariant::Valid};

$(
// SAFETY: The caller promised that `$atomic` and `$prim` have
Expand All @@ -436,28 +436,16 @@ mod atomics {
// the same size and bit validity.
unsafe impl<$($tyvar)?> TransmuteFrom<$prim, Valid, Valid> for $atomic {}

// SAFETY: The caller promised that `$atomic` and `$prim` have
// the same size.
unsafe impl<$($tyvar)?> SizeEq<$atomic> for $prim {
impl<$($tyvar)?> SizeCompat<$atomic> for $prim {
type CastFrom = $crate::pointer::cast::CastSized;
}
// SAFETY: See previous safety comment.
unsafe impl<$($tyvar)?> SizeEq<$prim> for $atomic {
impl<$($tyvar)?> SizeCompat<$prim> for $atomic {
type CastFrom = $crate::pointer::cast::CastSized;
}
// SAFETY: The caller promised that `$atomic` and `$prim` have
// the same size. `UnsafeCell<T>` has the same size as `T` [1].
//
// [1] Per https://doc.rust-lang.org/1.85.0/std/cell/struct.UnsafeCell.html#memory-layout:
//
// `UnsafeCell<T>` has the same in-memory representation as
// its inner type `T`. A consequence of this guarantee is that
// it is possible to convert between `T` and `UnsafeCell<T>`.
unsafe impl<$($tyvar)?> SizeEq<$atomic> for UnsafeCell<$prim> {
impl<$($tyvar)?> SizeCompat<$atomic> for UnsafeCell<$prim> {
type CastFrom = $crate::pointer::cast::CastSized;
}
// SAFETY: See previous safety comment.
unsafe impl<$($tyvar)?> SizeEq<UnsafeCell<$prim>> for $atomic {
impl<$($tyvar)?> SizeCompat<UnsafeCell<$prim>> for $atomic {
type CastFrom = $crate::pointer::cast::CastSized;
}

Expand Down
4 changes: 2 additions & 2 deletions src/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -674,7 +674,7 @@ macro_rules! try_transmute_ref {
// the value returned from this branch.
let u;

$crate::assert_size_eq!(t, u);
$crate::assert_size_compat!(t, u);
$crate::assert_align_gt_eq!(t, u);

Ok(&u)
Expand Down Expand Up @@ -783,7 +783,7 @@ macro_rules! try_transmute_mut {
// u)` as the value returned from this branch.
let u;

$crate::assert_size_eq!(t, u);
$crate::assert_size_compat!(t, u);
$crate::assert_align_gt_eq!(t, u);

Ok(&mut u)
Expand Down
19 changes: 10 additions & 9 deletions src/pointer/ptr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@ use crate::{
pointer::{
inner::PtrInner,
invariant::*,
transmute::{MutationCompatible, SizeEq, TransmuteFromPtr},
transmute::{MutationCompatible, SizeCompat, TransmuteFromPtr},
},
AlignmentError, CastError, CastType, KnownLayout, SizeError, TryFromBytes, ValidityError,
};
Expand Down Expand Up @@ -381,10 +381,10 @@ mod _conversions {
pub(crate) fn transmute<U, V, R>(self) -> Ptr<'a, U, (I::Aliasing, Unaligned, V)>
where
V: Validity,
U: TransmuteFromPtr<T, I::Aliasing, I::Validity, V, R> + SizeEq<T> + ?Sized,
U: TransmuteFromPtr<T, I::Aliasing, I::Validity, V, R> + SizeCompat<T> + ?Sized,
{
// SAFETY:
// - By `SizeEq::CastFrom: Cast`, `SizeEq::CastFrom` preserves
// - By `SizeCompat::CastFrom: Cast`, `SizeCompat::CastFrom` preserves
// referent address, and so we don't need to consider projections
// in the following safety arguments.
// - If aliasing is `Shared`, then by `U: TransmuteFromPtr<T>`, at
Expand All @@ -396,7 +396,7 @@ mod _conversions {
// operate on these references simultaneously
// - By `U: TransmuteFromPtr<T, I::Aliasing, I::Validity, V>`, it is
// sound to perform this transmute.
unsafe { self.project_transmute_unchecked::<_, _, <U as SizeEq<T>>::CastFrom>() }
unsafe { self.project_transmute_unchecked::<_, _, <U as SizeCompat<T>>::CastFrom>() }
}

#[doc(hidden)]
Expand All @@ -408,15 +408,16 @@ mod _conversions {
T: TransmuteFromPtr<T, I::Aliasing, I::Validity, V, R>,
{
// SAFETY:
// - By `SizeEq::CastFrom: Cast`, `SizeEq::CastFrom` preserves
// referent address, and so we don't need to consider projections
// in the following safety arguments.
// - By `SizeCompat::CastFrom: Cast`, `SizeCompat::CastFrom`
// preserves referent address, and so we don't need to consider
// projections in the following safety arguments.
// - It is trivially sound to have multiple `&T` referencing the
// same referent simultaneously
// - By `T: TransmuteFromPtr<T, I::Aliasing, I::Validity, V>`, it is
// sound to perform this transmute.
let ptr =
unsafe { self.project_transmute_unchecked::<_, _, <T as SizeEq<T>>::CastFrom>() };
let ptr = unsafe {
self.project_transmute_unchecked::<_, _, <T as SizeCompat<T>>::CastFrom>()
};
// SAFETY: `self` and `ptr` have the same address and referent type.
// Therefore, if `self` satisfies `I::Alignment`, then so does
// `ptr`.
Expand Down
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