Make it work for tuples of iterators?

[Boscop]

Would it be possible to support tuples of iterators? (Basically doing it per tuple component.)
So that this would work:

[taiki-e]

How does this work if the trait implemented for tuples?

[Boscop]

It would work per tuple constituent, for tuples where all constituents impl Iterator (considering that the proc-macro can't check the types of the constituent, it's ok to assume they impl Iterator).

[taiki-e]

Oh, sorry, I misunderstood that I could implement foreign traits for tuples.

[taiki-e]

Ok, I think we can support this based on the rules like the following:

• Traits are defined in the standard library and not implemented for tuples.
• All branches are considered to return tuples if at least one branch uses tuple syntax.

What is not clear yet is:

• Should macro generates enums per each tuple element or a single enum for all tuple elements? (i.e., return (EnumA, EnumB,) or return (EnumA, EnumA,))
• How do the above rules apply when multiple traits are specified?
• Should support complex syntax? (e.g., nested tuples)