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Improve soundness a bit by making TaggedArcPtr::try_as_arc_owned() unsafe

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Since the `ManuallyDrop` it returns can be safely used to consume the `Arc`, which is can cause UB if done incorrectly. See #18499.
This commit is contained in:
Chayim Refael Friedman 2024-12-04 03:38:37 +02:00
parent e6276c8b64
commit 3aeb5e66c4
1 changed files with 17 additions and 13 deletions
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30
crates/intern/src/symbol.rs
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@ -77,8 +77,12 @@ impl TaggedArcPtr {
}
/// Retrieves the tag.
///
/// # Safety
///
/// You can only drop the `Arc` if the instance is dropped.
#[inline]
pub(crate) fn try_as_arc_owned(self) -> Option<ManuallyDrop<Arc<Box<str>>>> {
pub(crate) unsafe fn try_as_arc_owned(self) -> Option<ManuallyDrop<Arc<Box<str>>>> {
// Unpack the tag from the alignment niche
let tag = Strict::addr(self.packed.as_ptr()) & Self::BOOL_BITS;
if tag != 0 {
@ -245,16 +249,14 @@ impl Symbol {
}
}
ManuallyDrop::into_inner(
match shard.raw_entry_mut().from_key_hashed_nocheck::<str>(hash, arc.as_ref()) {
RawEntryMut::Occupied(occ) => occ.remove_entry(),
RawEntryMut::Vacant(_) => unreachable!(),
}
.0
.0
.try_as_arc_owned()
.unwrap(),
);
let ptr = match shard.raw_entry_mut().from_key_hashed_nocheck::<str>(hash, arc.as_ref()) {
RawEntryMut::Occupied(occ) => occ.remove_entry(),
RawEntryMut::Vacant(_) => unreachable!(),
}
.0
.0;
// SAFETY: We're dropping, we have ownership.
ManuallyDrop::into_inner(unsafe { ptr.try_as_arc_owned().unwrap() });
debug_assert_eq!(Arc::count(arc), 1);
// Shrink the backing storage if the shard is less than 50% occupied.
@ -267,7 +269,8 @@ impl Symbol {
impl Drop for Symbol {
#[inline]
fn drop(&mut self) {
let Some(arc) = self.repr.try_as_arc_owned() else {
// SAFETY: We're dropping, we have ownership.
let Some(arc) = (unsafe { self.repr.try_as_arc_owned() }) else {
return;
};
// When the last `Ref` is dropped, remove the object from the global map.
@ -288,7 +291,8 @@ impl Clone for Symbol {
}
fn increase_arc_refcount(repr: TaggedArcPtr) -> TaggedArcPtr {
let Some(arc) = repr.try_as_arc_owned() else {
// SAFETY: We're not dropping the `Arc`.
let Some(arc) = (unsafe { repr.try_as_arc_owned() }) else {
return repr;
};
// increase the ref count