RUSTSEC-2026-0221: `event-listener` allows `!Send` tags to cross thread boundaries via `StackSlot`

> `event-listener` allows `!Send` tags to cross thread boundaries via `StackSlot`

| Details | |
| ------------------- | ---------------------------------------------- |
| Status | unsound |
| Package | `event-listener` |
| Version | `5.4.0` |
| URL | [https://github.com/smol-rs/event-listener/pull/163](https://github.com/smol-rs/event-listener/pull/163) |
| Date | 2026-07-13 |

Affected versions of `event-listener` unconditionally implement `Send` and
`Sync` for `StackSlot<'_, T>`, the stack-allocated listener type created
by the `listener!` macro.

This allows a `!Send` tag type set via `Event::with_tag` to be moved to
another thread and accessed via `StackSlot::wait`, causing a data race in safe
code.

See [advisory page](https://rustsec.org/advisories/RUSTSEC-2026-0221.html) for additional details.

Start with the linked advisory page and upstream event-listener pull request. Search this workspace for its event-listener dependency and check whether the affected 5.4.0 version is used; done means the workspace no longer relies on the vulnerable version and its dependency checks pass.

Written by the indexing model from the issue text.