Using `flat_map` on an `Option<T>` should suggest using `and_then` instead

[liquidev]

Code

fn main() {
    let opt = Some(1);
    opt.flat_map(|val| Some(val));
}

Current output

error[E0599]: `Option<{integer}>` is not an iterator
 --> src/main.rs:3:9
  |
3 |     opt.flat_map(|val| Some(val));
  |         ^^^^^^^^ `Option<{integer}>` is not an iterator
 --> /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/option.rs:518:1
  |
  = note: doesn't satisfy `Option<{integer}>: Iterator`
  |
  = note: the following trait bounds were not satisfied:
          `Option<{integer}>: Iterator`
          which is required by `&mut Option<{integer}>: Iterator`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `playground` due to previous error

Desired output

error[E0599]: `Option<{integer}>` is not an iterator
 --> src/main.rs:3:9
  |
3 |     opt.flat_map(|val| Some(val));
  |         ^^^^^^^^
  |         | `Option<{integer}>` is not an iterator
  |         = help: if you wish to map and flatten the option, use `and_then`
 --> /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/option.rs:518:1
  |
  = note: doesn't satisfy `Option<{integer}>: Iterator`
  |
  = note: the following trait bounds were not satisfied:
          `Option<{integer}>: Iterator`
          which is required by `&mut Option<{integer}>: Iterator`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `playground` due to previous error

Rationale and extra context

I've run into this confusing naming issue time and time again and I think the compiler could provide a more helpful error message in this case, hinting that you should use and_then instead of flat_map if you wish to do the same operation on options (which you could write as .map().flatten(), hence why I always think it's Option::flat_map and not Option::and_then.)

Other cases

No response

Anything else?

No response

[fmease]

#59450 related

[jieyouxu]

FWIW, since PR #112239 is merged, it should be quite easy to add this suggestion by marking and_then with #[rustc_confusables("flat_map")], along with a test.