Automatic-return type

Sometimes, the exact type of returned value is cumbersome to write out. In this case, #auto can be provided as the return type. It automatically determines the return type given the first return statement in the procedure.

// #auto would automatically determined to be:
//   Iterator(i32), bool, str
weird_return_type :: (x: i32) -> #auto {
    return iter.as_iter(1 .. 5) , false, "Hello, World!";

In some cases in Onyx, it is actually impossible to write the return type. #auto can be used in this case, and the compiler will figure out what type needs to be there. Look at this example from the standard library. :: (x: $I/Iterable, y: Iterator($Y)) -> #auto { ... } returns an iterator of pairs of the two values yielded from the left and right iterators. There is no way to write the return type, because you cannot spell the type of Iterator that x is because it is only Iterable, meaning you can call as_iter on it. Think about it, what could you write in Iterator(Pair(???, Y)) to make it correct?