`video_vulkan.rs` was fenced with the other thirteen GPU/FFI backends, but it never belonged with
them: its four sites are not ash calls, they are RAW POINTER DEREFERENCES inside the two FFmpeg
`lock_queue`/`unlock_queue` callback trampolines — exactly the case where the lint pays, and where a
proof carries an argument instead of restating a signature. Both bodies had none at all.
They keep `unsafe extern "C"`: FFmpeg invokes them with a `*mut AVHWDeviceContext`, which is a real
contract, now written down. What was missing is why the dereference chain is sound, and it is worth
stating because it is not obvious — the trampoline casts between pf_ffvk's `AVHWDeviceContext` and
ffmpeg-sys's (the same C struct declared twice), reads `user_opaque`, and dereferences it as a
`QueueLock`. That pointer borrows `VkCtxStorage::_queue_lock`, an `Arc<QueueLock>` whose field doc
already says it exists to outlive every call the context can make. The proof now connects those two
facts, so a future edit to the storage's lifetime has something to contradict.
With that, the file needs no exemption and the workspace `deny` covers it: 14 fenced files -> 13.
This is what removing a fence is supposed to look like — the sites that are worth narrowing get
narrowed, and the exemption disappears rather than being renewed.
Verified on .21: fmt + `clippy --workspace --all-targets -- -D warnings` + the feature-gated
`-p pf-encode` step, all rc=0 with no allow in the file.
enricobuehler
ed4bbc6b0b