Into iter

Suppose we wish get an iterator, and then return the result of the iterator. The into_iter function is helpful here—it converts the collection into an iterator.

Avoid errors

With into_iter() we can fix errors involving FromIterator. If we want u8, we need into_iter() to keep u8 instead of references to u8 values.

Example code

Consider this example code—the get_flattened_vec function returns a Vector of u8. To do this, we must use into_iter() and then call collect() on that iterator.

fn get_flattened_vec() -> Vec<u8> {
    // Create nested vectors.
    let vecs = vec![vec![], vec![]];
    // Use into iter to convert to an iterator in place, then flatten and collect.
    vecs.into_iter().flatten().collect()
}

fn main() {
    let result = get_flattened_vec();
    println!("{:?}", result);
}
[]

Error info

If we try to use iter() in the return statement, we will get a FromIterator error. It is important to consider the element types.

error[E0277]: a value of type Vec<u8> cannot be built from an iterator over elements of type &_
 --> src/main.rs:5:27...
  = help: the trait FromIterator<&_> is not implemented for Vec<u8>
  = help: the trait FromIterator<T> is implemented for Vec<T>
note: required by a bound in collect...

A summary

There is an important distinction between iter() and into_iter() in Rust. Iter will return references, while into_iter() will return the original types (which are needed for returns).