Better error message when forgetting iter() on slices

[edhebi]

Given the following code: https://play.rust-lang.org/?version=beta&mode=debug&edition=2018&gist=965472aeea871e8f49d3decac84abe64

fn get_slice() -> &'static [i32] {
    &[1, 2, 3, 4]
}

fn main() {
    let sqsum = get_slice().map(|i| i * i).sum();
}

The current output is:

error [E0599]: the method `map` exists for reference `&'static [i32]`, but its trait bounds were not satisfied
 --> src/main.rs:6:29
  |
6 |     let sqsum = get_slice().map(|i| i * i).sum();
  |                             ^^^ method cannot be called on `&'static [i32]` due to unsatisfied trait bounds
  |
  = note: the following trait bounds were not satisfied:
          `&'static [i32]: Iterator`
          which is required by `&mut &'static [i32]: Iterator`
          `[i32]: Iterator`
          which is required by `&mut [i32]: Iterator`

Ideally the output should at least mention that .iter() exists on slices