Using str::contains inside of a filter leads to confusing error message.

[ethercrow]

I've been trying to learn some Rust by doing CodeWars. I've spent some time being stuck on this diagnostic. One of the easiest exercises there is to count vowels in a string. First I did it with a loop, but then got stuck trying to rewrite is as a filter.

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=785ded94c58b6d9a3a8acd16145a4970

fn get_vowel_count(string: &str) -> usize {
        string
        .chars()
        .filter(|c| "aeiou".contains(c))
        .count()
}

The current output is:

error[E0277]: expected a `Fn<(char,)>` closure, found `char`
 --> src/lib.rs:4:38
  |
4 |         .filter(|c| "aeiou".contains(c))
  |                                      ^ expected an `Fn<(char,)>` closure, found `char`
  |
  = help: the trait `Fn<(char,)>` is not implemented for `char`
  = note: required because of the requirements on the impl of `FnOnce<(char,)>` for `&char`
  = note: required because of the requirements on the impl of `Pattern<'_>` for `&char`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `challenge`

Ideally the output should not lead with "expected an Fn" and also help me to add a missing ampersand.

[osa1]

and also help me to add a missing ampersand.

Do you mean a missing star? Working version:

fn get_vowel_count(string: &str) -> usize {
        string
        .chars()
        .filter(|c| "aeiou".contains(*c))
        .count()
}

I agree that the error message is very misleading.

Smaller repro:

fn test(c: &char) -> bool {
    "aeiou".contains(c)
}

[ethercrow]

Do you mean a missing star?

Both |c| "aeiou".contains(*c) and |&c| "aeiou".contains(c) are accepted. At the time of writing the ticket I found only the latter way to fix the error, that's why I said ampersand.

[edmorley]

This misleading diagnostic also caught me out too recently.

In my case the usage was virtually identical to the OP's:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=43fbdf70f0270c762d0558fdb34ae1f3

I also found #90970 which seems related.

[estebank]

Addressing in #91873