RUSTSEC-2026-0272 Panic-safety unsoundness in `Stack::pop`, `Fifo::pop_front` and `Value::replace_stable` (use-after-free / double-free)

Three removal and replacement paths destroy an initialized value before committing the metadata change that removes it. Destruction runs T::drop(), which is user code and may panic. If it does, the commit is skipped and the container still claims ownership of the already-destroyed value, so its own destructor drops that value a second time.

Stack::pop drops the top value before reducing next_ofs. Stack's destructor pops in a loop, so an unchanged next_ofs re-drops the same value.

Fifo::pop_front (via pop_front_inner) drops the front value before advancing read_pos. An unchanged read_pos still points at the destroyed value.

Value::replace_stable drops the existing value before writing the replacement. If the destructor panics the replacement is never written, but the backing storage still holds the old value's metadata.

This is separate from RUSTSEC-2021-0033, which covered a clone panic on insertion in push_cloned and was fixed in 0.6.1.

Impact

A heap-owning value is freed twice, corrupting the allocator. Reachable from safe Rust with any element type whose Drop can panic — no unsafe on the caller's side.

• CWE-415 (Double Free)
• CWE-416 (Use-After-Free)

Confirmed under AddressSanitizer, which reports attempting double-free on all three paths.

Fix

Fixed in 0.8.2.