Function with 

pub fn with<R: Send + 'static>(
    ctx: &Context,
    f: impl FnOnce(&mut Kernel) -> R + Send + 'static,
) -> (Synced, R)

Bring this thread’s mirror to ctx and run f on its kernel. The mirror is out of its slot while f runs, so a sync inside f builds its own and this one is put back after.