clippy suggests a change (use of iterator flatten) which it reports as an error if implemented

Summary

Given a loop that could be simplified using a flatten().iter().next() + if let Some() statement, clippy suggests an invalid solution, which it then suggests a correction for if implemented as suggested. The intermediate (incorrect) solution should be omitted.

Full context for the example bellow is at: https://github.com/hschimke/rxing/blob/4bd2e53a5c26cd40edb0337ed61ffe3bedacc4a6/src/pdf417/decoder/pdf_417_scanning_decoder.rs#L619

Reproducer

Given the below code:

for previousRowCodeword in detectionRXingResult
            .getDetectionRXingResultColumn(barcodeColumn as usize)
            .as_ref()
            .unwrap()
            .getCodewords()
        {
            // for (Codeword previousRowCodeword : detectionRXingResult.getDetectionRXingResultColumn(barcodeColumn).getCodewords()) {
            if let Some(previousRowCodeword) = previousRowCodeword {
                // if previousRowCodeword.is_some() {
                return ((if leftToRight {
                    previousRowCodeword.getEndX()
                } else {
                    previousRowCodeword.getStartX()
                }) as isize
                    + offset
                        * skippedColumns as isize
                        * (previousRowCodeword.getEndX() - previousRowCodeword.getStartX())
                            as isize) as u32;
            }
        }
        skippedColumns += 1;
    }

clippy suggests:

warning: unnecessary `if let` since only the `Some` variant of the iterator element is used
   --> src/pdf417/decoder/pdf_417_scanning_decoder.rs:619:9
    |
619 | /         for previousRowCodeword in detectionRXingResult
620 | |             .getDetectionRXingResultColumn(barcodeColumn as usize)
621 | |             .as_ref()
622 | |             .unwrap()
...   |
637 | |             }
638 | |         }
    | |_________^
    |
help: remove the `if let` statement in the for loop and then...
   --> src/pdf417/decoder/pdf_417_scanning_decoder.rs:626:13
    |
626 | /             if let Some(previousRowCodeword) = previousRowCodeword {
627 | |                 // if previousRowCodeword.is_some() {
628 | |                 return ((if leftToRight {
629 | |                     previousRowCodeword.getEndX()
...   |
636 | |                             as isize) as u32;
637 | |             }
    | |_____________^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_flatten
    = note: `#[warn(clippy::manual_flatten)]` on by default
help: try
    |
619 ~         for previousRowCodeword in detectionRXingResult
620 +             .getDetectionRXingResultColumn(barcodeColumn as usize)
621 +             .as_ref()
622 +             .unwrap()
623 +             .getCodewords().iter().flatten()
    |

If I implement the change such that the code looks like:

for previousRowCodeword in detectionRXingResult
            .getDetectionRXingResultColumn(barcodeColumn as usize)
            .as_ref()
            .unwrap()
            .getCodewords().iter().flatten()
        {
            // for (Codeword previousRowCodeword : detectionRXingResult.getDetectionRXingResultColumn(barcodeColumn).getCodewords()) {
            // if let Some(previousRowCodeword) = previousRowCodeword {
                // if previousRowCodeword.is_some() {
                return ((if leftToRight {
                    previousRowCodeword.getEndX()
                } else {
                    previousRowCodeword.getStartX()
                }) as isize
                    + offset
                        * skippedColumns as isize
                        * (previousRowCodeword.getEndX() - previousRowCodeword.getStartX())
                            as isize) as u32;
            // }
        }

This code does not necessarily make sense, but it is what was suggested by the lint. Rerunning clippy results in:

error: this loop never actually loops
   --> src/pdf417/decoder/pdf_417_scanning_decoder.rs:619:9
    |
619 | /         for previousRowCodeword in detectionRXingResult
620 | |             .getDetectionRXingResultColumn(barcodeColumn as usize)
621 | |             .as_ref()
622 | |             .unwrap()
...   |
637 | |             // }
638 | |         }
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#never_loop
    = note: `#[deny(clippy::never_loop)]` on by default
help: if you need the first element of the iterator, try writing
    |
619 ~         if let Some(previousRowCodeword) = detectionRXingResult
620 +             .getDetectionRXingResultColumn(barcodeColumn as usize)
621 +             .as_ref()
622 +             .unwrap()
623 +             .getCodewords().iter().flatten().next()
    |

Changing the code as suggested in the final error results in:

if let Some(previousRowCodeword) = detectionRXingResult
            .getDetectionRXingResultColumn(barcodeColumn as usize)
            .as_ref()
            .unwrap()
            .getCodewords().iter().flatten().next()
        {
            // for (Codeword previousRowCodeword : detectionRXingResult.getDetectionRXingResultColumn(barcodeColumn).getCodewords()) {
            // if let Some(previousRowCodeword) = previousRowCodeword {
                // if previousRowCodeword.is_some() {
                return ((if leftToRight {
                    previousRowCodeword.getEndX()
                } else {
                    previousRowCodeword.getStartX()
                }) as isize
                    + offset
                        * skippedColumns as isize
                        * (previousRowCodeword.getEndX() - previousRowCodeword.getStartX())
                            as isize) as u32;
            // }
        }

This is correct, and will execute as expected.

Version

`rustc 1.66.0 (69f9c33d7 2022-12-12)`
`cargo 1.66.0 (d65d197ad 2022-11-15)`
`clippy 0.1.66 (69f9c33d 2022-12-12)`

Additional Labels

No response

Start with the manual_flatten warning and reproduce the reported sequence using the Rust 1.66.0 example from src/pdf417/decoder/pdf_417_scanning_decoder.rs. Trace why applying its suggestion triggers never_loop, then verify that the intermediate invalid suggestion is omitted while the final if let ... flatten().next() guidance remains available.