Diagnostics: suggest using `array::IntoIter::new` to iterate over arrays

[yoshuawuyts]

When following the const-generics beta MVP blog post we ran into this diagnostics error. The blog post specifically requests filing const-generics related diagnostics issues, so here goes!

• Rust 1.51.0-beta.3 (2021-02-24)

Given the following code:

use std::array;
fn needs_vec(v: Vec<i32>) {
    // ...
}

let arr = [vec![0, 1], vec![1, 2, 3], vec![3]];
for elem in arr {
    needs_vec(elem);
}

The current output is:

error[E0277]: `[Vec<{integer}>; 3]` is not an iterator
 --> src/main.rs:8:17
  |
8 |     for elem in arr {
  |                 ^^^ borrow the array with `&` or call `.iter()` on it to iterate over it
  |
  = help: the trait `Iterator` is not implemented for `[Vec<{integer}>; 3]`
  = note: arrays are not iterators, but slices like the following are: `&[1, 2, 3]`
  = note: required because of the requirements on the impl of `IntoIterator` for `[Vec<{integer}>; 3]`
  = note: required by `into_iter`

Ideally the output should look like:

error[E0277]: `[Vec<{integer}>; 3]` is not an iterator
 --> src/main.rs:8:17
  |
8 |     for elem in arr {
  |                 ^^^ call `array::IntoIter::new(arr)` to iterate over the array
  |
  = help: the trait `Iterator` is not implemented for `[Vec<{integer}>; 3]`

[leonardo-m]

Isn't it better to wait for shorter ways to iterate an array by value?

[yoshuawuyts]

@leonardo-m it's unclear when, or even if such a method will land (though I certainly hope it will). With const generics scheduled to be stable on Rust 1.51.0 (four weeks from now) this seems like a diagnostics improvement which will have a material impact on people's use of Rust.