How can I construct and pass an iterator of iterators? - rust

Question

I am trying to grok Rust by implementing simple algorithms in it. I managed to make a generic merge_sorted, which ended up having the following signature:
fn merge_sorted<IL, ILL, I: Ord>(mut arrays: ILL) -> Vec<I>
where
IL: Iterator<Item = I>,
ILL: Iterator<Item = IL>,
{
// ...
}
This seems to be compiling on its own. The signature makes sense to me, as the function consumes the top-level iterator, and all the iterators it returns too. However, I am unable to construct a valid value to pass to this function:
fn main() {
let v1 = vec![1, 2];
let vectors = vec![v1.iter()];
merge_sorted(vectors.iter());
}
As expected, vectors in this sample has the type:
std::vec::Vec<std::slice::Iter<'_, i32>>
This is the error message I get:
error[E0277]: the trait bound `&std::slice::Iter<'_, {integer}>: std::iter::Iterator` is not satisfied
--> src\main.rs:58:5
|
58 | merge_sorted(vectors.iter());
| ^^^^^^^^^^^^ `&std::slice::Iter<'_, {integer}>` is not an iterator; maybe try calling `.iter()` or a similar method
|
= help: the trait `std::iter::Iterator` is not implemented for `&std::slice::Iter<'_, {integer}>`
note: required by `merge_sorted`
Where does the & come from?

Answer 1

Vec::iter borrows the items it contains, so you are iterating over borrowed iterators (&std::slice::Iter) that do not implement Iterator. To consume a vector in order to have the ownership of the items, you must call Vec::into_iter:
fn main() {
let v1 = vec![1, 2];
let vectors = vec![v1.iter()]; // You can use `into_iter` there to iterate over ints.
merge_sorted(vectors.into_iter());
}
You can also require IntoIterators that can make easier the usage of your API:
fn merge_sorted<IterT, IterIterT, T: Ord>(mut arrays: IterIterT) -> Vec<T>
where
IterT: IntoIterator<Item = T>,
IterIterT: IntoIterator<Item = IterT>,
{
panic!();
}
fn main() {
let v1 = vec![1, 2];
let vectors = vec![v1];
merge_sorted(vectors);
}