[confusing error message] iterate over str

[gnzlbg]

Given:

fn main() {
    for c in "foobarbaz" {
        println!("{}", c);
    }
}

Rust produces (playground):

error[E0277]: the trait bound `&str: std::iter::Iterator` is not satisfied
 --> src/main.rs:2:5
  |
2 | /     for c in "foobarbaz" {
3 | |         println!("{}", c);
4 | |     }
  | |_____^ `&str` is not an iterator; maybe try calling `.iter()` or a similar method
  |
  = help: the trait `std::iter::Iterator` is not implemented for `&str`
  = note: required by `std::iter::IntoIterator::into_iter`

The suggestion says iter or a similar method, but &str doesn't have an .iter() method (that's why it says "or a similar method").

I don't think this is good enough.

It should scan the methods of str that return something that implements Iterator (or can be auto-deref to something that implements it) and suggest those. In this case, chars(), bytes(),... The error message should look more like this:

error[E0277]: the trait bound `&str: std::iter::Iterator` is not satisfied
 --> src/main.rs:2:5
  |
2 | /     for c in "foobarbaz" {
3 | |         println!("{}", c);
4 | |     }
  | |_____^ `&str` is not an iterator
  |
  = help: the trait `std::iter::Iterator` is not implemented for `&str`
  = note: required by `std::iter::IntoIterator::into_iter`

The following methods of `&str` return an `Iterator`:
  - `chars`
  - `bytes`
  - ... 

[GuillaumeGomez]

Sounds like an extension of the very controversial methods suggestion. But why not? If other people agree on it, I can add it.

[zackmdavis]

(Given that for loops automatically invoke .into_iter(), it's not clear that we should ever recommend .iter() here. (That is, most of the time this "maybe try calling .iter() or a similar method" message appears, .iter() won't exist, because if it did, the type likely already implements IntoIterator too and we wouldn't be in this situation.))

[zackmdavis]

The message comes from rustc_on_unimplemented, so this would seem to be dependent on #44755.

[est31]

I would like such a message

[estebank]

error[E0277]: the trait bound `&str: std::iter::Iterator` is not satisfied
 --> src/main.rs:2:14
  |
2 |     for c in "foobarbaz" {
  |              ^^^^^^^^^^^ `&str` is not an iterator; try calling `.chars()` or `.bytes()`
  |
  = help: the trait `std::iter::Iterator` is not implemented for `&str`
  = note: required by `std::iter::IntoIterator::into_iter`