Method join

src/thread_pool.rs:710–718

(&'static self, a: A, b: B)

        //   `Runnable`.
        //
        //   Vacuously true, since `Self: 'static`.
        //
        // * If `schedule_task` is `!Send`/`!Sync`, all of the `Runnable`'s
        //   `Waker`s must be used and dropped on the original thread.
        //
        //   Vacuously true, since `S: Scheduler` is `Send + Sync` so too
        //   must `schedule_task` be `Send + Sync`.
        //
        // * If `schedule_task` is `!'static`, borrowed variables must outlive
        //   all of the `Runnable`'s `Waker`s.
        //
        //   Vacuously true, since `S: 'static`.