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Make Ptr::try_with_unchecked safe #2953

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c7adbc8
Update `Cargo.lock`
joshlf Jan 30, 2026
01118b6
Make `Ptr::try_with_unchecked` safe
joshlf Jan 30, 2026
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4 changes: 2 additions & 2 deletions Cargo.lock
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58 changes: 21 additions & 37 deletions src/pointer/ptr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -916,28 +916,29 @@ mod _casts {

/// Attempts to transform the pointer, restoring the original on
/// failure.
///
/// # Safety
///
/// If `I::Aliasing != Shared`, then if `f` returns `Err(err)`, no copy
/// of `f`'s argument must exist outside of `err`.
#[inline(always)]
pub(crate) unsafe fn try_with_unchecked<U, J, E, F>(
self,
f: F,
) -> Result<Ptr<'a, U, J>, E::Mapped>
pub(crate) fn try_with<U, J, E, F>(self, f: F) -> Result<Ptr<'a, U, J>, E::Mapped>
where
U: 'a + ?Sized,
J: Invariants<Aliasing = I::Aliasing>,
E: TryWithError<Self>,
F: FnOnce(Ptr<'a, T, I>) -> Result<Ptr<'a, U, J>, E>,
// TODO: The invariance issue is caused by `E` not being bound by
// `'b`, which thus requires it to be `'static`. If we can somehow
// polyfill a GAT so that it's actually `E<'b>` here, then we should
// be able to solve the `'static` issue. That will, in turn, remove
// the need for us to add `.map_err(|err| err.map_src(drop))` to
// many `f` closures.
F: for<'b> FnOnce(Ptr<'b, T, I>) -> Result<Ptr<'b, U, J>, E>,
{
let old_inner = self.as_inner();
#[rustfmt::skip]
let res = f(self).map_err(#[inline(always)] move |err: E| {
err.map(#[inline(always)] |src| {
drop(src);

// TODO: Mention the `for<'b>` variance on `F` to prove that
// `f` cannot make another copy of `self`.

// SAFETY:
// 0. Aliasing is either `Shared` or `Exclusive`:
// - If aliasing is `Shared`, then it cannot violate
Expand Down Expand Up @@ -968,21 +969,6 @@ mod _casts {
});
res
}

/// Attempts to transform the pointer, restoring the original on
/// failure.
pub(crate) fn try_with<U, J, E, F>(self, f: F) -> Result<Ptr<'a, U, J>, E::Mapped>
where
U: 'a + ?Sized,
J: Invariants<Aliasing = I::Aliasing>,
E: TryWithError<Self>,
F: FnOnce(Ptr<'a, T, I>) -> Result<Ptr<'a, U, J>, E>,
I: Invariants<Aliasing = Shared>,
{
// SAFETY: `I::Aliasing = Shared`, so the safety condition does not
// apply.
unsafe { self.try_with_unchecked(f) }
}
}

/// # Safety
Expand Down Expand Up @@ -1162,19 +1148,17 @@ mod _casts {
U: 'a + ?Sized + KnownLayout + Read<I::Aliasing, R>,
[u8]: Read<I::Aliasing, R>,
{
// SAFETY: The provided closure returns the only copy of `slf`.
unsafe {
self.try_with_unchecked(|slf| match slf.try_cast_into(CastType::Prefix, meta) {
Ok((slf, remainder)) => {
if remainder.len() == 0 {
Ok(slf)
} else {
Err(CastError::Size(SizeError::<_, U>::new(())))
}
#[rustfmt::skip]
return self.try_with(#[inline(always)] |slf| match slf.try_cast_into(CastType::Prefix, meta) {
Ok((slf, remainder)) => {
if remainder.len() == 0 {
Ok(slf)
} else {
Err(CastError::Size(SizeError::<_, U>::new(())))
}
Err(err) => Err(err.map_src(|_slf| ())),
})
}
}
Err(err) => Err(err.map_src(|_slf| ())),
});
}
}

Expand Down
14 changes: 6 additions & 8 deletions src/util/macro_util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -584,7 +584,7 @@ where
let res = ptr.try_with(#[inline(always)] |ptr| {
let ptr = ptr.recall_validity::<Initialized, _>();
let ptr = ptr.cast::<_, crate::layout::CastFrom<Dst>, _>();
ptr.try_into_valid()
ptr.try_into_valid().map_err(|err| err.map_src(drop))
});
match res {
Ok(ptr) => {
Expand Down Expand Up @@ -643,13 +643,11 @@ where
let ptr = Ptr::from_mut(self.0);
// SAFETY: The provided closure returns the only copy of `ptr`.
#[rustfmt::skip]
let res = unsafe {
ptr.try_with_unchecked(#[inline(always)] |ptr| {
let ptr = ptr.recall_validity::<Initialized, (_, (_, _))>();
let ptr = ptr.cast::<_, crate::layout::CastFrom<Dst>, _>();
ptr.try_into_valid()
})
};
let res = ptr.try_with(#[inline(always)] |ptr| {
let ptr = ptr.recall_validity::<Initialized, (_, (_, _))>();
let ptr = ptr.cast::<_, crate::layout::CastFrom<Dst>, _>();
ptr.try_into_valid().map_err(|err| err.map_src(drop))
});
match res {
Ok(ptr) => {
static_assert!(Src: ?Sized + KnownLayout, Dst: ?Sized + KnownLayout => {
Expand Down
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