Cannot index into a value of type `std::fs::ReadDir`

Question

I'm trying to get the path to the first entry in a directory, but it seems the read_dir response type ReadDir is an iterator and cannot be indexed, e.g.

fn get_single_file_or_none(dir_path: &PathBuf) -> Result<Option<PathBuf>, io::Error> {
    let entries = fs::read_dir(dir_path)?;

    if entries.count() == 1 {
        let path = entries[0].path();
//                        ^^ cannot index into a value of type `std::fs::ReadDir`
        return Ok(path);
    }

    return Ok(None);
}

How can I get the path to the first (and in this case, the only) entry in the directory?

I cant start iterating over the entries, knowing there's only one, and return the first one I hit. Is that the only solution? e.g.

if entries.count() == 1 {
    for entry_res in entries {
        return match entry_res {
            Err(er) => Err(er),
            Ok(en) => Ok(Some(en.path())),
        };
    }
}

Answer 1

itertools has the handy exactly_one() function:

use itertools::Itertools;

fn get_single_file_or_none(dir_path: &PathBuf) -> Result<Option<PathBuf>, io::Error> {
    let entries = fs::read_dir(dir_path)?;
    Ok(entries.exactly_one().ok().transpose()?.map(|it| it.path()))
}

This is more efficient than count(), since it will only read the directory once.