//! Compile-time assertions that the public `WaitGroup` types are `Send + Sync`.
//!
//! These types are meant to be passed across threads and shared among them.
//! This file guards against accidental regressions — if someone later adds a
//! `!Send` or `!Sync` field (e.g. `Rc`, `RefCell`, a raw pointer), the build
//! will fail here instead of at user sites.
const