Better suggestion when attempting to iterate over array

[frewsxcv]

Code

let a = [1, 2, 4, 5, 6];

for e in a {
    println!("{}", e);
}

Compiler error

error[E0277]: the trait bound `[{integer}; 5]: std::iter::Iterator` is not satisfied
 --> <anon>:6:5
  |
6 |     for e in a {
  |     ^ trait `[{integer}; 5]: std::iter::Iterator` not satisfied
  |
  = note: `[{integer}; 5]` is not an iterator; maybe try calling `.iter()` or a similar method
  = note: required by `std::iter::IntoIterator::into_iter`

error: aborting due to previous error

Suggestion

Instead of suggesting .iter(), it'd be cool if we cool if we could check if the borrowed type implements IntoIterator and then suggest prefixing with &. More concretely, in the example above, [T; 5] does not implement IntoIterator but &[T; 5] does.

[durka]

For reference the message is generated here. Improving it would require expanding the #[rustc_on_unimplemented] machinery or abandoning it for a custom message somewhere in the compiler (maybe in the for loop lowering but that might be too early). Iterator and IntoIterator aren't lang items but arguably the latter should be.

[nagisa]

for i in [0; N].iter() {}

Works for arrays of any size N, so the suggestion is valid? Whether you opt into explicitly converting into slice and using its IntoIterator or use the slice’s iter/iter_mut method through coercions is purely stylistic choice.

[Mark-Simulacrum]

The span in this case is also quite bad -- probably because of the desugaring, I'd guess.

[frewsxcv]

Greetings from three years in the future 👋🏻

If anyone stumbles across this, arrays now implement IntoIterator, so this shouldn't be an issue anymore https://blog.rust-lang.org/2021/05/11/edition-2021.html#intoiterator-for-arrays