It is sometimes useful to wrap a Guard and a &V into a single type instead of requiring a Guard as input. We could add a Ref wrapper type to support this. See the advanced lifetimes guide for context.

I'm unsure how the API should look as I don't want this to be the default return type for get. We could potentially add a Ref::create method that takes a guard and a closure that creates a value tied to the guard and extends the lifetime. However, we have to be careful about not creating self references. Though this is unlikely, it would be possible to create a HashMap<K, &Guard> or similar through Ref::create, so we might have to unfortunately restrict it to 'static types.