Add single-element tuple for `collect_tuple`

[amab8901]

I'm trying to select exactly one element via let the_one = vector.iter().filter(|item| item == condition).collect_tuple(). It needs to match exactly one element, no more, no less. If 2 elements are in the iterator after the filter, then that's an error (due to ambiguity on which item to use later in the code).

The .first() and .last() and .next() methods on iterators don't work because they don't guarantee that there's only one element in the vector/iterator.

[Philippe-Cholet]

What about Itertools::exactly_one?

let the_one = vector.iter().exactly_one().map_err(|it| ..)?;

[phimuemue]

For the sake of completeness: 1-element tuples are supported, but you may need type annotations:

    let vector = vec![1,2,3,4];
    let the_one : Option<(_,)> = dbg!(vector.iter().filter(|item| **item >= 2).collect_tuple());
    //                     ^
    //                     |
    //                     +-- Note the comma to signify a 1-element tuple.

[amab8901]

What about Itertools::exactly_one?

let the_one = vector.iter().exactly_one().map_err(|it| ..)?;

oh, I didn't see that one, thanks!