dyn-safety / dyn-friendlyness

[danielhenrymantilla]

The current workaround used by nougat (that is, the : for<'any> LendingIterator__Item super trait which "only" features the specific associated type) does not bode well with dyn Traits.

Mainly: dyn Traits require that their associated types be provided, but to my knowledge it is not possible to higher-order specify the associated type of a super trait.

That is:

• dyn for<'any> LendingIterator<T = &'any u8> fails saying that 'any is unbound / an unknown lifetime parameter;
• dyn LendingIterator + for<'any> LendingIterator__Item<T = …> fails because it combines two full traits under a dyn;
• dyn for<'any> LendingIterator__Item<T = …> may actually work (as in, it's a ?Sized implementor of LendingIterator), but, alas, it is no dyn LendingIterator;