Struct total_order_multi_map::SyncSendHelper

pub struct SyncSendHelper<K, V> { /* fields omitted */ }

Delegate the job of deciding about Send, Sync to rustc (ignore this)

only the *const V::Target is not default Send/Sync in TotalOrderMultiMap as it's a pointer, but we can ignore it as whenever we accessed a value through it we can argue that we could have accessed the value "just" through safe code. It would just have been slower. And using the fast path doesn't circumvent any safety mechanisms like e.g. lock guards. As such if this struct is Send/Sync than TotalOrderMultiMap can be Send/Sync, too.