TupleCollect for N = 0

[ivan770]

Currently, it's impossible to use next_tuple and similar tuple methods for unit type, even though such an impl could just return empty tuple on each call, without advancing inner iterator.

[scottmcm]

Can you say more about why this would be useful to you? I feel like I'd most often want the compiler error here, because if I'm using next_tuple I'm expecting to advance the iterator. Accidentally getting an infinite iterator of units -- which I can't think of why I'd ever want as an adapter, and iter::repeat(()) exists if needed -- feels like it'd be easily error-prone.

[ivan770]

Can you say more about why this would be useful to you? I feel like I'd most often want the compiler error here, because if I'm using next_tuple I'm expecting to advance the iterator. Accidentally getting an infinite iterator of units -- which I can't think of why I'd ever want as an adapter, and iter::repeat(()) exists if needed -- feels like it'd be easily error-prone.

At first I wanted to use unit tuples as a placeholder for "Callable that doesn't require any args" in API like this: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=eea1cf14d62513501165abca7fcf11e0. However, the whole idea was wrong and eventually API got changed, since there is also a problem with specifying the item type here: https://github.com/rust-itertools/itertools/blob/master/src/tuple_impl.rs#L271. Quite unsure now if it's even implementable without something like never-type.