rust - How to iterate over and filter an array?

Question

I'm trying to write a program that involves filtering and folding over arrays. I've been using The Rust Programming Language, first edition as a reference, but I don't understand what happens when I form iterators over arrays. Here is an example:

fn compiles() {
    let range = (1..6);
    let range_iter = range.into_iter();
    range_iter.filter(|&x| x == 2);
}

fn does_not_compile() {
    let array = [1, 4, 3, 2, 2];
    let array_iter = array.into_iter();
    //13:34 error: the trait `core::cmp::PartialEq<_>` is not implemented for the type `&_` [E0277]
    array_iter.filter(|&x| x == 2);
}

fn janky_workaround() {
    let array = [1, 4, 3, 2, 2];
    let array_iter = array.into_iter();
    // Note the dereference in the lambda body
    array_iter.filter(|&x| *x == 2);
}

(Rust playground)

In the first function, I follow that the iterator over the range does not take ownership, so I must take a &x in filter's lambda, but I don't understand why the second example with the array behaves differently.

Answer 1

In cases like this, it's very useful to force the compiler to tell you the type of the variable. Let's trigger a type error by assigning the closure argument to an incompatible type:

array_iter.filter(|x| { let _: () = x; x == 2 });

This fails with:

error[E0308]: mismatched types
 --> src/lib.rs:4:41
  |
4 |     array_iter.filter(|x| { let _: () = x; x == 2 });
  |                                    --   ^ expected `()`, found `&&{integer}`
  |                                    |
  |                                    expected due to this

Now we know the type of x is a &&{integer} - a reference to a reference to some kind of integer. We can then match against that instead:

fn hooray() {
    let array = [1, 4, 3, 2, 2];
    let array_iter = array.into_iter();
    array_iter.filter(|&&x| x == 2);
}

The question now becomes "why is it a reference to a reference"? The short version is that the iterator of an array returns references (see the type Item = &'a T part). In addition, Iterator::filter passes a reference to the closure to prevent moving and subsequently losing non-Copy types.

In Rust 1.51, you can use array::IntoIter to get a by-value iterator:

fn hooray() {
    let array = [1, 4, 3, 2, 2];
    let array_iter = std::array::IntoIter::new(array);
    array_iter.filter(|&x| x == 2);
}