FN single_match_else

[matthiaskrgr]

clippy 0.0.212 (b2601be 2018-11-27)

I had this code which did not trigger any lints:

    let input = match directory {
        Some(value) => value,
        None => {
            return Err((
                ErrorKind::RemoveDirNoArg,
                "No argument assigned to --remove-dir, example: 'git-repos,registry-sources'"
                    .to_string(),
            ))
        }
    };

After applying rustfmt, an additional ; was added:

    let input = match directory {
        Some(value) => value,
        None => {
            return Err((
                ErrorKind::RemoveDirNoArg,
                "No argument assigned to --remove-dir, example: 'git-repos,registry-sources'"
                    .to_string(),
            ));                                   // <--------- HERE
        }
    };

which caused clippy::single_match_else to trigger.

warning: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
   --> src/library.rs:432:17
    |
432 |       let input = match directory {
    |  _________________^
433 | |         Some(value) => value,
434 | |         None => {
435 | |             return Err((
...   |
440 | |         }
441 | |     };
    | |_____^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
help: try this
    |
432 |     let input = if let Some(value) = directory { value } else {
433 |     return Err((
434 |         ErrorKind::RemoveDirNoArg,
435 |         "No argument assigned to --remove-dir, example: 'git-repos,registry-sources'"
436 |             .to_string(),
437 |     ));
  ...

I think the link should also trigger on the unformatted example!

I also prefer using ok_or_else and ? to the suggestion.

    let input = directory.ok_or_else(|| Err(
        ErrorKind::RemoveDirNoArg,
        "No argument assigned to --remove-dir, example: 'git-repos,registry-sources'".to_string(),
    ))?;

Looking at this again

    match Some(1) => {
        Some(_) => {...}
        None => { return 1 },
    }

is equivalent to

    match Some(1) => {
        Some(_) => {...}
        None => return 1,
    }

which is a common pattern and shouldn't be linted IMO.

If anything is changed, I think clippy shouldn't lint for the second pattern either.

[tnielens]

This lint currently doesn't lint cases where the "else" part of the 2 arm match is a single statement/expression. The bug lies in that { return } is considered as a single expression whereas { return; } isn't.

One approach would be to consider single statement blocks equivalently. That would join @mikerite's last comment. That is, the three following examples wouldn't be linted.

    match Some(1) => {
        Some(_) => {...}
        None => return 1,
    }

    match Some(1) => {
        Some(_) => {...}
        None => { return 1 },
    }

// this case is currently linted

    match Some(1) => {
        Some(_) => {...}
        None => { return 1; },
    }