[swift-evolution] [Pitch] Remove destructive consumption from Sequence

[Brent Royal-Gordon]

> This use case seems interesting enough to at least deserve some consideration before we decide to require iterators to have reference semantics.  IMO the fact that `next` is a mutating requirement provides a pretty strong indication of the destructive / consuming semantics of using an iterator.  Requiring reference semantics gives up flexibility and I’m not completely convinced it adds clarity (but still keeping an open mind about this).

My feeling is that, except for the trivial case of IndexingIterator and other iterators over collections, all iterators *should* need reference semantics. If your iterator can offer value semantics, then you should convert it into a Collection to gain the benefits of indices. If it can't offer value semantics, then you should write it as a class so those reference semantics are obvious. This would get rid of the really ugly caveat that's always festered at the heart of Iterator:

>>> Obtain each separate iterator from separate calls to the sequence's makeIterator() method rather than by copying. Copying an iterator is safe, but advancing one copy of an iterator by calling its next() method may invalidate other copies of that iterator. for-in loops are safe in this regard.

The underlying cause of this requirement is that you can't know whether a given iterator is a value type or a reference type. Let's fix that.

-- 
Brent Royal-Gordon
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